Add support for sendmmsg(2) on linux #4374

Job	Run time
Check nightly-only targets (nightly)	25s
Check --no-default-features (nightly)	20s
Check selected Tier 3 platforms (nightly)	59s
Check (stable)	1m 17s
Rustfmt	17s
Check (nightly)	1m 19s
Test rustix_use_experimental_asm (powerpc64le-linux)	55s
Check (1.63)	1m 34s
Test rustix_use_experimental_asm (s390x-linux)	1m 6s
Test (ubuntu)	42s
Test use-libc (ubuntu)	29s
Test (ubuntu-20.04)	41s
Test use-libc (i686-linux)	45s
Test (i686-linux)	1m 34s
Test use-libc (aarch64-linux)	44s
Test (aarch64-linux)	2m 18s
Test use-libc (powerpc64le-linux)	44s
Test (powerpc64le-linux)	1m 8s
Test use-libc (riscv64-linux)	40s
Test (riscv64-linux)	1m 45s
Test use-libc (s390x-linux)	45s
Test (s390x-linux)	1m 45s
Test use-libc (arm-linux)	46s
Test (arm-linux)	1m 31s
Test (ubuntu-stable)	46s
Test (i686-linux-stable)	1m 1s
Test (aarch64-linux-stable)	1m 4s
Test (riscv64-linux-stable)	1m 6s
Test (s390x-linux-stable)	1m 47s
Test (powerpc64le-linux-stable)	1m 26s
Test (arm-linux-stable)	1m 55s
Test (ubuntu-1.63)	2m 1s
Test (i686-linux-1.63)	2m 18s
Test (aarch64-linux-1.63)	2m 23s
Test (riscv64-linux-1.63)	2m 20s
Test (s390x-linux-1.63)	2m 58s
Test (powerpc64le-linux-1.63)	2m 16s
Test (arm-linux-1.63)	2m 37s
Test (macos-latest)	44s
Test (macos-12)	54s
Test (windows)	40s
Test (windows-2019)	45s
Test (musl)	47s
	54m 17s

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