Add `shm_open`/`shm_unlink` (POSIX shared memory) #3320

Job	Run time
Check nightly-only targets (nightly)	32s
Check --no-default-features (nightly)	1m 13s
Check selected Tier 3 platforms (nightly)	6m 52s
Rustfmt	13s
Check (stable)	11m 7s
Check (nightly)	8m 50s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 37s
Check (1.63)	14m 52s
Test use-libc (ubuntu)	1m 4s
Test (ubuntu)	1m 5s
Test use-libc (i686-linux)	1m 31s
Test (ubuntu-20.04)	1m 7s
Test use-libc (aarch64-linux)	1m 28s
Test (i686-linux)	1m 24s
Test use-libc (powerpc64le-linux)	2m 6s
Test (aarch64-linux)	1m 35s
Test use-libc (riscv64-linux)	1m 48s
Test (powerpc64le-linux)	1m 41s
Test use-libc (arm-linux)	1m 22s
Test (riscv64-linux)	1m 26s
Test (s390x-linux)	1m 45s
Test (arm-linux)	1m 27s
Test (ubuntu-stable)	59s
Test (i686-linux-stable)	1m 22s
Test (aarch64-linux-stable)	1m 39s
Test (riscv64-linux-stable)	1m 22s
Test (s390x-linux-stable)	1m 36s
Test (powerpc64le-linux-stable)	1m 22s
Test (arm-linux-stable)	1m 39s
Test (ubuntu-1.63)	5m 7s
Test (i686-linux-1.63)	4m 37s
Test (aarch64-linux-1.63)	4m 59s
Test (riscv64-linux-1.63)	5m 38s
Test (s390x-linux-1.63)	4m 40s
Test (powerpc64le-linux-1.63)	6m 3s
Test (arm-linux-1.63)	4m 47s
Test (macos-latest)	1m 15s
Test (macos-11)	1m 22s
Test (windows)	1m 11s
Test (windows-2019)	1m 13s
	1h 56m 56s

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