[PowerPC] Save a dword in the powerpc64 signal trampoline

[bdragon]

Summary

In rS291668, an instruction was added to sigcode64.S without the nop pad at the end being taken out.

Due to alignment, this means that a dword is being wasted on the shared page for no reason.

Take out this nop, and add some comments while I'm here.

[bdragon]

sys/powerpc/powerpc/sigcode64.S

70 ↗

(On Diff #66417)

Actually this comment is incorrect now that I remember what I was thinking at the time regarding it being exec_copyout_strings() that cares.

When I was tracing through this, I was confused about how it worked. Apparently the code in exec_copyout_strings() is only used when a unified shared page is not used.

The rest of the comment should be accurate though.

[bdragon]

OK, so my comments are incorrect in general. The nop still needs to go though.

[bdragon]

Remove incorrect comments from original patch, add comment to describe why we're exiting instead.