Add `rustix::process::exit` to terminate processes #4150

Job	Run time
Check --no-default-features (nightly)	54s
Check (stable)	4m 0s
Check nightly-only targets (nightly)	23s
Rustfmt	4s
Check (nightly)	4m 20s
Check selected Tier 3 platforms (nightly)	2m 24s
Check (1.63)	7m 46s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 6s
Test (ubuntu)	44s
Test use-libc (ubuntu)	36s
Test (ubuntu-20.04)	43s
Test use-libc (i686-linux)	1m 13s
Test (i686-linux)	1m 1s
Test use-libc (aarch64-linux)	1m 18s
Test (aarch64-linux)	1m 12s
Test use-libc (powerpc64le-linux)	1m 34s
Test (powerpc64le-linux)	1m 8s
Test use-libc (riscv64-linux)	1m 38s
Test (riscv64-linux)	1m 4s
Test use-libc (arm-linux)	1m 20s
Test (s390x-linux)	1m 18s
Test (arm-linux)	1m 2s
Test (ubuntu-stable)	40s
Test (i686-linux-stable)	1m 27s
Test (aarch64-linux-stable)	1m 30s
Test (riscv64-linux-stable)	1m 0s
Test (s390x-linux-stable)	1m 11s
Test (powerpc64le-linux-stable)	1m 11s
Test (arm-linux-stable)	1m 4s
Test (ubuntu-1.63)	3m 11s
Test (i686-linux-1.63)	3m 30s
Test (aarch64-linux-1.63)	3m 55s
Test (riscv64-linux-1.63)	3m 59s
Test (s390x-linux-1.63)	3m 48s
Test (powerpc64le-linux-1.63)	3m 37s
Test (arm-linux-1.63)	3m 53s
Test (macos-latest)	18m 5s
Test (macos-12)	1m 15s
Test (windows)	46s
Test (windows-2019)	57s
Test (musl)	37s
	1h 32m 24s

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