[POWERPC] msun: fix incorrect flag in fesetexceptflag

Fix incorrect mask being used when FE_INVALID bit is wanted by user.
The problem was noticed thanks to msun fenv tests.

Reviewed by: jhibbits, luporl
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D27201