powerpc: Sync icache on SIGILL, in case of cache issues

The update of jemalloc to 5.1.0 exposed a cache syncing issue on a Freescale
e500 base system. There was already code in the FPU emulator to address
this, but it was limited to a single static variable, and did not attempt to
sync the cache. This pulls that out to the higher level program exception
handler, and syncs the cache.

If a SIGILL is hit a second time at the same address, it will be treated as
a real illegal instruction, and handled accordingly.