Add `sched_getscheduler` & `sched_setscheduler` #4384

Job	Run time
Check (stable)	1m 21s
Check (nightly)	1m 28s
Check nightly-only targets (nightly)	17s
Check selected Tier 3 platforms (nightly)	31s
Check --no-default-features (nightly)	17s
Check (1.63)	1m 38s
Rustfmt	12s
Test rustix_use_experimental_asm (powerpc64le-linux)	46s
Test use-libc (ubuntu)	23s
Test rustix_use_experimental_asm (s390x-linux)	39s
Test use-libc (i686-linux)	25s
Test (ubuntu)	25s
Test use-libc (aarch64-linux)	27s
Test (ubuntu-20.04)	26s
Test use-libc (powerpc64le-linux)	36s
Test (i686-linux)	37s
Test use-libc (riscv64-linux)	27s
Test (aarch64-linux)	40s
Test use-libc (s390x-linux)	34s
Test (powerpc64le-linux)	37s
Test use-libc (arm-linux)	33s
Test (riscv64-linux)	40s
Test (s390x-linux)	32s
Test (arm-linux)	36s
Test (ubuntu-stable)	26s
Test (i686-linux-stable)	41s
Test (aarch64-linux-stable)	35s
Test (riscv64-linux-stable)	40s
Test (s390x-linux-stable)	40s
Test (powerpc64le-linux-stable)	29s
Test (arm-linux-stable)	42s
Test (ubuntu-1.63)	39s
Test (i686-linux-1.63)	31s
Test (aarch64-linux-1.63)	33s
Test (riscv64-linux-1.63)	41s
Test (s390x-linux-1.63)	33s
Test (powerpc64le-linux-1.63)	43s
Test (arm-linux-1.63)	44s
Test (macos-latest)	25s
Test (macos-12)	43s
Test (windows)	28s
Test (windows-2019)	29s
Test (musl)	28s
	26m 17s

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