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.

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