Add a `rustix::fs::ABS` constant. #4371

Job	Run time
Check selected Tier 3 platforms (nightly)	4m 49s
Check --no-default-features (nightly)	1m 2s
Check nightly-only targets (nightly)	27s
Check (stable)	6m 5s
Rustfmt	18s
Check (nightly)	6m 12s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 51s
Check (1.63)	10m 25s
Test rustix_use_experimental_asm (s390x-linux)	1m 14s
Test (ubuntu)	46s
Test use-libc (ubuntu)	47s
Test (ubuntu-20.04)	43s
Test use-libc (i686-linux)	1m 48s
Test (i686-linux)	1m 34s
Test use-libc (aarch64-linux)	1m 43s
Test (aarch64-linux)	1m 26s
Test use-libc (powerpc64le-linux)	1m 51s
Test (powerpc64le-linux)	1m 19s
Test use-libc (riscv64-linux)	1m 21s
Test (riscv64-linux)	1m 5s
Test use-libc (s390x-linux)	1m 41s
Test (s390x-linux)	1m 48s
Test use-libc (arm-linux)	1m 12s
Test (arm-linux)	1m 23s
Test (ubuntu-stable)	53s
Test (i686-linux-stable)	1m 35s
Test (aarch64-linux-stable)	1m 27s
Test (riscv64-linux-stable)	1m 15s
Test (s390x-linux-stable)	1m 35s
Test (powerpc64le-linux-stable)	1m 29s
Test (arm-linux-stable)	1m 11s
Test (ubuntu-1.63)	1m 57s
Test (i686-linux-1.63)	2m 51s
Test (aarch64-linux-1.63)	2m 50s
Test (riscv64-linux-1.63)	2m 47s
Test (s390x-linux-1.63)	2m 44s
Test (powerpc64le-linux-1.63)	2m 21s
Test (arm-linux-1.63)	2m 25s
Test (macos-latest)	39s
Test (macos-12)	54s
Test (windows)	42s
Test (windows-2019)	58s
Test (musl)	46s
	1h 24m 9s

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