Fix p{read,write}v{,v2}'s encoding of the offset argument on Linux. (… #3507

Job	Run time
Check nightly-only targets (nightly)	29s
Check --no-default-features (nightly)	19s
Check (stable)	4m 20s
Check selected Tier 3 platforms (nightly)	3m 7s
Check (nightly)	3m 45s
Rustfmt	6s
Check (1.48)	11m 36s
Test rustix_use_experimental_asm (powerpc64le-linux)	3m 8s
Test use-libc (ubuntu)	47s
Test (ubuntu)	45s
Test use-libc (i686-linux)	1m 8s
Test (ubuntu-20.04)	1m 0s
Test use-libc (aarch64-linux)	3m 32s
Test (i686-linux)	1m 28s
Test use-libc (powerpc64le-linux)	3m 15s
Test (aarch64-linux)	3m 43s
Test use-libc (arm-linux)	3m 37s
Test (powerpc64le-linux)	3m 45s
Test (s390x-linux)	3m 40s
Test (arm-linux)	4m 8s
Test (ubuntu-stable)	52s
Test (ubuntu-1.48)	5m 56s
Test (i686-linux-stable)	1m 3s
Test (aarch64-linux-stable)	3m 49s
Test (s390x-linux-stable)	2m 54s
Test (powerpc64le-linux-stable)	3m 48s
Test (arm-linux-stable)	3m 34s
Test (ubuntu-1.48)	6m 40s
Test (i686-linux-1.48)	6m 44s
Test (aarch64-linux-1.48)	10m 8s
Test (s390x-linux-1.48)	9m 7s
Test (powerpc64le-linux-1.48)	8m 47s
Test (arm-linux-1.48)	9m 32s
Test (macos-latest)	45s
Test (macos-11)	59s
Test (windows)	1m 9s
Test (windows-2019)	1m 24s
	2h 14m 49s

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