Fix more missing `cfg(feature = "process")`. #3598

Job	Run time
Check --no-default-features (nightly)	1m 11s
Check nightly-only targets (nightly)	21s
Rustfmt	6s
Check selected Tier 3 platforms (nightly)	3m 58s
Check (stable)	5m 50s
Check (nightly)	5m 58s
Test (ubuntu)	1m 3s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 7s
Check (1.63)	11m 41s
Test (ubuntu-20.04)	46s
Test use-libc (ubuntu)	39s
Test (i686-linux)	1m 42s
Test use-libc (i686-linux)	1m 13s
Test (aarch64-linux)	1m 59s
Test use-libc (aarch64-linux)	1m 11s
Test (powerpc64le-linux)	1m 12s
Test use-libc (powerpc64le-linux)	2m 12s
Test use-libc (riscv64-linux)	1m 48s
Test (riscv64-linux)	2m 1s
Test (s390x-linux)	1m 45s
Test use-libc (arm-linux)	1m 7s
Test (arm-linux)	1m 44s
Test (ubuntu-stable)	39s
Test (i686-linux-stable)	1m 18s
Test (aarch64-linux-stable)	1m 0s
Test (riscv64-linux-stable)	1m 5s
Test (s390x-linux-stable)	1m 38s
Test (powerpc64le-linux-stable)	1m 56s
Test (arm-linux-stable)	1m 9s
Test (ubuntu-1.63)	4m 15s
Test (i686-linux-1.63)	4m 31s
Test (aarch64-linux-1.63)	6m 54s
Test (riscv64-linux-1.63)	4m 50s
Test (s390x-linux-1.63)	6m 45s
Test (powerpc64le-linux-1.63)	4m 59s
Test (arm-linux-1.63)	4m 43s
Test (macos-latest)	1m 33s
Test (macos-11)	1m 10s
Test (windows)	45s
Test (windows-2019)	1m 10s
	1h 40m 54s

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