7034139: G1: assert(Thread::current()->is_ConcurrentGC_thread()) failed: only a conc GC thread can call this. Summary: We were calling STS join and leave during a STW pause and we are not suppoesed to. I now only call those during concurrent phase. I also added stress code in the non-product builds to force an overflows (the condition that ws uncovering the bug) to make sure it does not happen again. Reviewed-by: johnc, brutisso

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# The copied fdlibm routines in sharedRuntimeTrig.o must not be optimized
OPT_CFLAGS/sharedRuntimeTrig.o = $(OPT_CFLAGS/NOOPT)

# Must also specify if CPU is big endian
CFLAGS += -DVM_BIG_ENDIAN