[PPC64] Fix GDB sigtramp detection

Current implementation of ppcfbsd_pc_in_sigtramp() seems to take only 32-bit
PowerPC in account, as on 64-bit PowerPC most kernel instruction addresses will
be wrongly reported as in sigtramp.

This change adds proper sigtramp detection for PPC64.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D22199