branches: 1.2.4;
t_execregs: On aarch64, make sure to align stack to 16 bytes.

Should avoid SIGBUS with strict alignment (SCTLR_EL0.A bit).

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