t_signal_and_sp: Test makecontext and pthread_create stack alignment.

PR kern/59327: user stack pointer is not aligned properly

Test whether exec/spawn will zero registers.

Currently implemented only for a handful of architectures; should
extend this to all the others, and extend as appropriate if we find
more register content is worth testing (like maybe vector registers,
but they are managed differently anyway and less likely to leak).

VAX test contributed (and tested) by Kalvis Duckmanton, with some
tweaks by me; the others written and tested by me. IA64 skipped,
even though I suspect it _would_ leak if the kernel code ran as is,
because I have no way to test it.

PR kern/59084: exec/spawn leaks register content

Test whether exec/spawn will zero registers.

Currently implemented only for a handful of architectures; should
extend this to all the others, and extend as appropriate if we find
more register content is worth testing (like maybe vector registers,
but they are managed differently anyway and less likely to leak).

VAX test contributed (and tested) by Kalvis Duckmanton, with some
tweaks by me; the others written and tested by me. IA64 skipped,
even though I suspect it _would_ leak if the kernel code ran as is,
because I have no way to test it.

PR kern/59084: exec/spawn leaks register content

t_signal_and_sp: Check sp on elf constructor/destructor entry too.

XXX Should maybe test both .ctors/.dtors and .init/fini_array, but
for now I'm limiting this to whatever gcc uses by default for each
architecture.

PR kern/59327: user stack pointer is not aligned properly

Test stack pointer alignment in various scenarios.

1. elf entry point
2. main function
3. signal handler

Extend the test to amd64 while here -- fortunately both aarch64 and
amd64 pass, but others, such as mips, will fail:

PR kern/59327: user stack pointer is not aligned properly

This extends the test that was previously written for:

PR kern/58149: aarch64: Cannot return from a signal handler if SP was
misaligned when the signal arrived

With any luck, this will help us to systematically eradicate misaligned
stack pointers as hypothesized to be the reason for:

PR port-mips/59236: Multiple segfaults in erlite3 boot

Test whether exec/spawn will zero registers.

Currently implemented only for a handful of architectures; should
extend this to all the others, and extend as appropriate if we find
more register content is worth testing (like maybe vector registers,
but they are managed differently anyway and less likely to leak).

VAX test contributed (and tested) by Kalvis Duckmanton, with some
tweaks by me; the others written and tested by me. IA64 skipped,
even though I suspect it _would_ leak if the kernel code ran as is,
because I have no way to test it.

PR kern/59084: exec/spawn leaks register content

Test stack pointer alignment in various scenarios.

1. elf entry point
2. main function
3. signal handler

Extend the test to amd64 while here -- fortunately both aarch64 and
amd64 pass, but others, such as mips, will fail:

PR kern/59327: user stack pointer is not aligned properly

This extends the test that was previously written for:

PR kern/58149: aarch64: Cannot return from a signal handler if SP was
misaligned when the signal arrived

With any luck, this will help us to systematically eradicate misaligned
stack pointers as hypothesized to be the reason for:

PR port-mips/59236: Multiple segfaults in erlite3 boot

Test stack pointer alignment in various scenarios.

1. elf entry point
2. main function
3. signal handler

Extend the test to amd64 while here -- fortunately both aarch64 and
amd64 pass, but others, such as mips, will fail:

PR kern/59327: user stack pointer is not aligned properly

This extends the test that was previously written for:

PR kern/58149: aarch64: Cannot return from a signal handler if SP was
misaligned when the signal arrived

With any luck, this will help us to systematically eradicate misaligned
stack pointers as hypothesized to be the reason for:

PR port-mips/59236: Multiple segfaults in erlite3 boot

t_signal_and_sp: Fix threadsp test to actually test the thread sp.

Copypasta error had it testing alignment of signalsp instead, and
since it was always null, that always passed. Mark it xfail on mips
now as originally expected.

While here, deal with some other issues:

- Test was failing on riscv for me because I haven't updated this
`current' VM in a while so it didn't have the fix for 57721
(pthread_attr_setstack incorrectly adjusts address as if for guard
page). Don't mark it xfail.

- Fix amd64 threadspfunc.S to adjust rsp like in all the other amd64
stubs so it's congruent to 0 mod 16, not congruent to 8 mod 16.

- Fix hppa contextspfunc.S to use separate registers for separate
purposes at the same time, instead of expecting addil to preserve
%r1 AND yield a result we use later in %r1.

PR kern/59327: user stack pointer is not aligned properly

t_signal_and_sp: Test makecontext and pthread_create stack alignment.

PR kern/59327: user stack pointer is not aligned properly