Fix errors with char signedness. #3757

Job	Run time
Check nightly-only targets (nightly)	21s
Rustfmt	3s
Check selected Tier 3 platforms (nightly)	3m 31s
Check (stable)	5m 53s
Check --no-default-features (nightly)	53s
Check (nightly)	6m 20s
Check (1.63)	10m 37s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 2s
Test (ubuntu)	44s
Test use-libc (ubuntu)	37s
Test (ubuntu-20.04)	43s
Test use-libc (i686-linux)	50s
Test (i686-linux)	1m 4s
Test use-libc (aarch64-linux)	57s
Test (aarch64-linux)	1m 4s
Test use-libc (powerpc64le-linux)	57s
Test (powerpc64le-linux)	1m 2s
Test use-libc (riscv64-linux)	56s
Test (riscv64-linux)	1m 1s
Test use-libc (arm-linux)	56s
Test (s390x-linux)	1m 2s
Test (arm-linux)	1m 2s
Test (ubuntu-stable)	37s
Test (i686-linux-stable)	52s
Test (aarch64-linux-stable)	59s
Test (riscv64-linux-stable)	56s
Test (s390x-linux-stable)	1m 9s
Test (powerpc64le-linux-stable)	57s
Test (arm-linux-stable)	57s
Test (ubuntu-1.63)	2m 46s
Test (i686-linux-1.63)	2m 55s
Test (aarch64-linux-1.63)	2m 58s
Test (riscv64-linux-1.63)	2m 51s
Test (s390x-linux-1.63)	2m 59s
Test (powerpc64le-linux-1.63)	2m 59s
Test (arm-linux-1.63)	3m 13s
Test (macos-latest)	1m 10s
Test (macos-11)	1m 22s
Test (windows)	49s
Test (windows-2019)	49s
Test (musl)	43s
	1h 13m 36s

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