Add a `rustix::fs::ABS` constant. #4359

Job	Run time
Check nightly-only targets (nightly)	24s
Check --no-default-features (nightly)	54s
Check selected Tier 3 platforms (nightly)	34s
Check (stable)	1m 13s
Rustfmt	7s
Check (nightly)	1m 19s
Check (1.63)	1m 26s
Test rustix_use_experimental_asm (powerpc64le-linux)	46s
Test rustix_use_experimental_asm (s390x-linux)	59s
Test (ubuntu)	32s
Test (ubuntu-20.04)	31s
Test use-libc (ubuntu)	24s
Test (i686-linux)	41s
Test use-libc (i686-linux)	42s
Test (aarch64-linux)	41s
Test use-libc (aarch64-linux)	41s
Test (powerpc64le-linux)	27s
Test use-libc (powerpc64le-linux)	37s
Test (riscv64-linux)	34s
Test use-libc (riscv64-linux)	42s
Test (s390x-linux)	35s
Test use-libc (s390x-linux)	41s
Test (arm-linux)	41s
Test use-libc (arm-linux)	40s
Test (ubuntu-stable)	26s
Test (i686-linux-stable)	39s
Test (aarch64-linux-stable)	40s
Test (riscv64-linux-stable)	39s
Test (s390x-linux-stable)	27s
Test (powerpc64le-linux-stable)	29s
Test (arm-linux-stable)	39s
Test (ubuntu-1.63)	37s
Test (i686-linux-1.63)	28s
Test (aarch64-linux-1.63)	37s
Test (riscv64-linux-1.63)	29s
Test (s390x-linux-1.63)	27s
Test (powerpc64le-linux-1.63)	29s
Test (arm-linux-1.63)	41s
Test (macos-latest)	33s
Test (macos-12)	28s
Test (windows)	33s
Test (windows-2019)	24s
Test (musl)	25s
	27m 1s

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