Add support for sendmmsg(2) on linux #4380

Job	Run time
Check --no-default-features (nightly)	57s
Check selected Tier 3 platforms (nightly)	5m 10s
Check (stable)	6m 7s
Rustfmt	16s
Check (nightly)	6m 10s
Check nightly-only targets (nightly)	22s
Check (1.63)	9m 31s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 24s
Test rustix_use_experimental_asm (s390x-linux)	1m 8s
Test (ubuntu)	42s
Test use-libc (ubuntu)	43s
Test (ubuntu-20.04)	44s
Test use-libc (i686-linux)	58s
Test (i686-linux)	58s
Test use-libc (aarch64-linux)	1m 8s
Test (aarch64-linux)	1m 17s
Test use-libc (powerpc64le-linux)	1m 45s
Test (powerpc64le-linux)	1m 6s
Test use-libc (riscv64-linux)	1m 16s
Test (riscv64-linux)	1m 26s
Test use-libc (s390x-linux)	1m 6s
Test (s390x-linux)	1m 4s
Test use-libc (arm-linux)	1m 5s
Test (arm-linux)	1m 16s
Test (ubuntu-stable)	48s
Test (i686-linux-stable)	1m 19s
Test (aarch64-linux-stable)	1m 23s
Test (riscv64-linux-stable)	1m 27s
Test (s390x-linux-stable)	1m 1s
Test (powerpc64le-linux-stable)	1m 26s
Test (arm-linux-stable)	1m 18s
Test (ubuntu-1.63)	1m 57s
Test (i686-linux-1.63)	2m 17s
Test (aarch64-linux-1.63)	2m 39s
Test (riscv64-linux-1.63)	2m 51s
Test (s390x-linux-1.63)	2m 30s
Test (powerpc64le-linux-1.63)	2m 32s
Test (arm-linux-1.63)	2m 15s
Test (macos-latest)	36s
Test (macos-12)	50s
Test (windows)	53s
Test (windows-2019)	46s
Test (musl)	45s
	1h 17m 12s

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