Add `sched_getscheduler` & `sched_setscheduler` #4395

Job	Run time
Check nightly-only targets (nightly)	21s
Check selected Tier 3 platforms (nightly)	4m 54s
Check --no-default-features (nightly)	53s
Check (stable)	6m 11s
Rustfmt	4s
Check (nightly)	6m 12s
Test rustix_use_experimental_asm (powerpc64le-linux)	59s
Check (1.63)	9m 58s
Test rustix_use_experimental_asm (s390x-linux)	58s
Test (ubuntu)	42s
Test (ubuntu-20.04)	46s
Test use-libc (ubuntu)	43s
Test (i686-linux)	59s
Test use-libc (i686-linux)	1m 0s
Test (aarch64-linux)	1m 4s
Test use-libc (aarch64-linux)	1m 0s
Test (powerpc64le-linux)	1m 3s
Test use-libc (powerpc64le-linux)	1m 1s
Test (riscv64-linux)	1m 1s
Test (s390x-linux)	1m 4s
Test use-libc (riscv64-linux)	1m 3s
Test (arm-linux)	1m 3s
Test use-libc (s390x-linux)	1m 0s
Test (ubuntu-stable)	47s
Test use-libc (arm-linux)	1m 6s
Test (i686-linux-stable)	1m 1s
Test (aarch64-linux-stable)	1m 1s
Test (riscv64-linux-stable)	56s
Test (s390x-linux-stable)	59s
Test (powerpc64le-linux-stable)	1m 10s
Test (arm-linux-stable)	1m 12s
Test (ubuntu-1.63)	2m 3s
Test (i686-linux-1.63)	2m 21s
Test (aarch64-linux-1.63)	2m 29s
Test (riscv64-linux-1.63)	2m 26s
Test (s390x-linux-1.63)	2m 33s
Test (powerpc64le-linux-1.63)	2m 23s
Test (arm-linux-1.63)	2m 19s
Test (macos-latest)	48s
Test (macos-12)	52s
Test (windows)	51s
Test (windows-2019)	47s
Test (musl)	46s
	1h 12m 49s

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