Add support for sendmmsg(2) on linux #4292

Job	Run time
Check --no-default-features (nightly)	55s
Check (stable)	3m 36s
Check (nightly)	3m 31s
Check selected Tier 3 platforms (nightly)	4m 47s
Check nightly-only targets (nightly)	25s
Check (1.63)	3m 47s
Rustfmt	11s
Test rustix_use_experimental_asm (powerpc64le-linux)	56s
Test use-libc (ubuntu)	40s
Test (ubuntu)	37s
Test use-libc (i686-linux)	57s
Test (ubuntu-20.04)	31s
Test use-libc (aarch64-linux)	58s
Test (i686-linux)	39s
Test use-libc (powerpc64le-linux)	49s
Test (aarch64-linux)	39s
Test use-libc (riscv64-linux)	52s
Test (powerpc64le-linux)	46s
Test use-libc (arm-linux)	53s
Test (riscv64-linux)	46s
Test (s390x-linux)	49s
Test (arm-linux)	49s
Test (ubuntu-stable)	33s
Test (i686-linux-stable)	37s
Test (aarch64-linux-stable)	48s
Test (riscv64-linux-stable)	42s
Test (s390x-linux-stable)	35s
Test (powerpc64le-linux-stable)	46s
Test (arm-linux-stable)	48s
Test (ubuntu-1.63)	40s
Test (i686-linux-1.63)	49s
Test (aarch64-linux-1.63)	43s
Test (riscv64-linux-1.63)	47s
Test (s390x-linux-1.63)	48s
Test (powerpc64le-linux-1.63)	48s
Test (arm-linux-1.63)	39s
Test (macos-latest)	33s
Test (macos-12)	48s
Test (windows)	32s
Test (windows-2019)	35s
Test (musl)	48s
	42m 12s

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