issue #3996: impl gettid for macOS & cygwin64. #3187

Job	Run time
utest-regression-blackbox-test	5m 1s
multiple-arch-aarch64	5m 27s
multiple-arch-armv7	5m 34s
cygwin64-cache	2m 48s
cygwin64	6m 46s
fast	1s
build-centos7	2m 14s
build-cross-aarch64	2m 21s
build-ubuntu16	1m 32s
build-cross-arm	2m 27s
build-ubuntu18	1m 34s
build-ubuntu20	1m 46s
coverage	2m 34s
multiple-arch-amd64	1m 20s
test-done	1s
	41m 26s

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