exceptor: Add SA_ONSTACK flag the POSIX backend #1587

Job	Run time
gir	35s
manylinux (x86)	2m 46s
cross (windows-x86_64-mingw, --host=x86_64-w64-mingw32 --without-prebuilds=sdk:host, g++-mingw-w6...	4m 4s
manylinux (x86_64)	2m 23s
cross (windows-x86-mingw, --host=i686-w64-mingw32 --without-prebuilds=sdk:host, g++-mingw-w64-i686)	3m 31s
manylinux (x86_64-musl)	2m 11s
mobile (ios-arm64)	2m 23s
cross (linux-mips, --host=mips-linux-gnu, g++-mips-linux-gnu)	1m 33s
manylinux (armhf)	2m 5s
mobile (android-x86)	1m 46s
cross (linux-mipsel, --host=mipsel-linux-gnu, g++-mipsel-linux-gnu)	1m 22s
manylinux (arm64)	2m 12s
mobile (android-x86_64)	1m 45s
cross (linux-mips64, --host=mips64-linux-gnuabi64, g++-mips64-linux-gnuabi64)	1m 34s
manylinux (arm64-musl)	2m 10s
cross (linux-mips64el, --host=mips64el-linux-gnuabi64, g++-mips64el-linux-gnuabi64)	1m 22s
mobile (android-arm)	1m 50s
manylinux (mips)	1m 38s
manylinux (mipsel)	1m 38s
mobile (android-arm64)	6m 37s
manylinux (mips64)	1m 42s
manylinux (mips64el)	1m 44s
native (windows-x86_64, "windows-latest")	3m 15s
qnx-armeabi	3m 11s
native (windows-x86, "windows-latest")	3m 5s
native (macos-x86_64, "macos-13")	3m 44s
native (macos-arm64, "macos-latest")	1m 40s
native (linux-x86_64, "ubuntu-latest")	2m 1s
native (linux-x86, "ubuntu-latest")	2m 17s
native (freebsd-arm64, ["self-hosted", "freebsd", "arm64"])	5m 33s
	1h 13m 37s

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