[PowerPC] Fix *context on ELFv2

Due to ELFv1 specific code in _ctx_start.S and makecontext.c, userspace
context switching was completely broken on ELFv2.

With this change, we now pass the libthr test suite.

Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D22421