arm: asm.h - change GOT_INIT to take the normal asm label

Don't pass the label to use for the "current address" as the macro
parameter. It obscures its purpose and makes the use of local labels
extremely unobvious, so you are forced to invent a name that is mostly
useless otherwise:

GOT_INIT(r3, .Lgot.execsp_start, .Lpc.execsp_start)
GOT_INITSYM(.Lgot.execsp_start, .Lpc.execsp_start)
vs.
0: GOT_INIT(r3, .Lgot.execsp_start)
GOT_INITSYM(.Lgot.execsp_start, 0b)

Same object code is generated.

Make main() actually return zero. From Riastradh.
PR kern/59327: user stack pointer is not aligned properly

t_signal_and_sp arm/execsp.S: Try using registers that aren't in use.

The first draft of this was instead using all the registers that
_are_ in use on entry to the elf entry point, which understandably
confused __start when we jumped to it after trashing all its inputs.

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

t_signal_and_sp: Draft add arm support.

Compile-tested only, let's see how this goes.

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