gdb: only return signal values for powerpc's gdb_cpu_signal()

Summary:
The values returned by this function are reported to the gdb client as
the reason for the break in execution, a signal value such as SIGTRAP,
SIGEMT, or SIGSEGV. As such, exact vector numbers can be misidentified.
Return SIGEMT in the default case instead.

Reviewed by: alfredo
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28046