PowerPC safepoints

PowerPC safepoints

"I saw it almost pass tests once.  Ship it."

Rough PowerPC port of sb-safepoint and related features.  Occasional
issues remain (see comment in test suite).

  - Sprinkle fake_foreign_function_call fairy dust over safepoint code.

  - Use the CSP page for synchronization, but not for its value.

  - No altstack, hence no tricks to escape from it.

  - Remove workarounds related to the SIGALRM handler; but keep
    SIGINT happy for now, with a somewhat less questionable check.