Add `shm_open`/`shm_unlink` (POSIX shared memory) #3278

Job	Run time
Check (stable)	4m 16s
Check (nightly)	4m 10s
Rustfmt	15s
Check (1.63)	4m 19s
Check nightly-only targets (nightly)	27s
Check selected Tier 3 platforms (nightly)	3m 49s
Check --no-default-features (nightly)	1m 14s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 31s
Test (ubuntu)	1m 3s
Test use-libc (ubuntu)	1m 9s
Test (ubuntu-20.04)	1m 4s
Test use-libc (i686-linux)	1m 45s
Test (i686-linux)	1m 9s
Test use-libc (aarch64-linux)	2m 14s
Test (aarch64-linux)	1m 10s
Test use-libc (powerpc64le-linux)	2m 0s
Test (powerpc64le-linux)	1m 7s
Test use-libc (riscv64-linux)	1m 37s
Test (riscv64-linux)	1m 7s
Test use-libc (arm-linux)	2m 11s
Test (s390x-linux)	1m 11s
Test (arm-linux)	1m 12s
Test (ubuntu-stable)	1m 8s
Test (i686-linux-stable)	1m 7s
Test (aarch64-linux-stable)	1m 13s
Test (riscv64-linux-stable)	1m 10s
Test (s390x-linux-stable)	1m 11s
Test (powerpc64le-linux-stable)	1m 9s
Test (arm-linux-stable)	1m 10s
Test (ubuntu-1.63)	1m 8s
Test (i686-linux-1.63)	1m 9s
Test (aarch64-linux-1.63)	1m 9s
Test (riscv64-linux-1.63)	1m 10s
Test (s390x-linux-1.63)	1m 8s
Test (powerpc64le-linux-1.63)	1m 10s
Test (arm-linux-1.63)	1m 11s
Test (macos-latest)	1m 5s
Test (macos-11)	50s
Test (windows)	53s
Test (windows-2019)	56s
	59m 57s

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