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

[mhorne]

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.

For reference, the list of gdb signal values:
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=include/gdb/signals.def

[mhorne]

Ping. Any opposition to this patch?

[alfredo]

@mhorne looks good to me. I also look on other platforms and your patch makes sense.