[POWERPC] Floating-Point Exception trap support

Add support for Floating-Point Exception traps on 32 and 64 bit platforms.
Also make sure to clean FPSCR on EXEC and thread exit

Author of initial version: Renato Riolino <renato.riolino@eldorad.org.br>

Reviewed by: jhibbits
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D23623