Multithreaded gctest terminated with signal SIGTRAP on cygwin

[ivmai]

Source: master (2b45fef5d)
Host: Windows x64 (cygwin64)
How to reproduce: gcc -O0 -g -I include -Wall -Wextra -Wpedantic -D GC_ASSERTIONS -D USE_MMAP -D USE_MUNMAP -D GC_THREADS tests/gctest.c extra/gc.c && gdb a.exe
Output:
Program terminated with signal SIGTRAP, Trace/breakpoint trap.

Also, reproduced w/o gdb (silent termination of gctest), after:

--> Marking for collection #122 after 8295584 allocated bytes
Marked from 34 dirty pages
World-stopped marking took 14 ms 628300 ns (9 ms in average)

Related #523

[ivmai]

Source: master (7f1503d)
Build: https://ci.appveyor.com/project/ivmai/bdwgc/builds/50339350/job/luq90myko01fvw7y
Config: TARGET=msys64, CFLAGS_EXTRA=-O3 -march=native -D GC_THREADS -D GC_GCJ_SUPPORT -D GC_TIME_LIMIT=10 -D WINXP_USE_PERF_COUNTER -D NO_MSGBOX_ON_ERROR -D NO_TEST_ENDTHREADEX
Note: msys2 is a clone of cygwin (i.e. it is not a mingw build)
Output (gctest):

Supported VDBs: manual mprotect
Switched to incremental mode
Emulating dirty bits with mprotect/signals (covering also ptr-free pages)

Build is successful because gctest is terminated silently with zero exit code.

Relates to #634