[PowerPC] add machdep.uprintf_signal sysctl

Add support for sysctl 'machdep.uprintf_signal' that prints debugging
information on trap signal.

Reviewed by: jhibbits, luporl, bdragon
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D26004