Support `SIGRTMIN+n` signals. #4683

Job	Run time
Check nightly-only targets (nightly)	25s
Check (stable)	2m 46s
Check --no-default-features (nightly)	56s
Check (nightly)	2m 47s
Check selected Tier 3 platforms (nightly)	35s
Rustfmt	11s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 15s
Check (1.63)	2m 53s
Test rustix_use_experimental_asm (s390x-linux)	2m 35s
Test (ubuntu)	31s
Test use-libc (ubuntu)	50s
Test (ubuntu-20.04)	33s
Test (i686-linux)	34s
Test use-libc (i686-linux)	2m 24s
Test use-libc (aarch64-linux)	1m 7s
Test (aarch64-linux)	34s
Test use-libc (powerpc64le-linux)	2m 16s
Test (powerpc64le-linux)	34s
Test use-libc (riscv64-linux)	2m 8s
Test (riscv64-linux)	34s
Test use-libc (s390x-linux)	2m 15s
Test (s390x-linux)	32s
Test use-libc (arm-linux)	2m 17s
Test (arm-linux)	33s
Test (ubuntu-stable)	33s
Test (i686-linux-stable)	33s
Test (aarch64-linux-stable)	33s
Test (riscv64-linux-stable)	33s
Test (s390x-linux-stable)	32s
Test (powerpc64le-linux-stable)	33s
Test (arm-linux-stable)	35s
Test (ubuntu-1.63)	32s
Test (i686-linux-1.63)	34s
Test (aarch64-linux-1.63)	36s
Test (riscv64-linux-1.63)	31s
Test (s390x-linux-1.63)	33s
Test (powerpc64le-linux-1.63)	32s
Test (arm-linux-1.63)	34s
Test (macos-latest)	17s
Test (macos-13)	34s
Test (windows)	18s
Test (windows-2019)	20s
Test (musl)	33s
	41m 51s

Provide feedback