1.0.25.28: always use SIG_RESUME_FROM_GC

1.0.25.28: always use SIG_RESUME_FROM_GC

The other mechanism relied on real time signals which made it freeze
when the sysystem wide real time signal queue got full on Linux. A
full queue spells trouble for other processes using rt signals.

All platforms are changed to use SIGUSR1 and SIGUSR2 for
SIG_STOP_FOR_GC and SIG_RESUME_FROM_GC.

Check that SIG_RESUME_FROM_GC is never signalled without a
corresponding sigwait.