[PowerPC] Save a dword in the powerpc64 signal trampoline

In r291668, 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.

Reviewed by: jhibbits
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D23055