cmake: pre-fill `HAVE_STDATOMIC_H`, `HAVE_ATOMIC` for mingw-w64 #1465

Job	Run time
cygwin, AM x86_64 openssl R	8m 31s
linux-mingw, AM gcc	2m 42s
cygwin, CM x86_64 openssl	7m 51s
linux-mingw, CM gcc	1m 20s
dl-mingw, CM 9.5.0-x86_64 schannel	11m 20s
dl-mingw, CM 7.3.0-x86_64 schannel U	7m 53s
msvc, CM x64-windows schannel MultiSSL U	8m 10s
msys2, AM x86_64 !proxy	7m 36s
msvc, CM x64-windows openssl	9m 34s
dl-mingw, CM 6.4.0-i686 schannel !unity	3m 16s
msys2, AM x86_64 default	4m 52s
msvc, CM x64-uwp openssl	4m 32s
msys2, CM x86_64 default	6m 15s
msvc, CM x64-windows libressl	8m 49s
msys2, AM x86_64 default R	7m 37s
msvc, CM x64-windows boringssl	7m 42s
mingw, AM x86_64 default	6m 46s
msvc, CM x64-windows wolfssl	8m 55s
mingw, AM x86_64 c-ares U	24m 58s
msvc, CM x64-windows mbedtls	7m 48s
mingw, CM x86_64 schannel c-ares U	6m 54s
mingw, CM ucrt-x86_64 schannel R TrackMemory	2m 49s
mingw, CM clang-x86_64 openssl	2m 6s
mingw, CM ucrt-x86_64 schannel uwp	2m 41s
mingw, CM x86_64 schannel dev debug	2m 39s
mingw, CM i686 schannel R	2m 34s
	2h 56m 10s

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