Fix the type of `_RawOpcode` on uclibc. #3221

Job	Run time
Check --no-default-features (nightly)	1m 22s
Check nightly-only targets (nightly)	31s
Check (stable)	8m 49s
Check selected Tier 3 platforms (nightly)	4m 42s
Rustfmt	4s
Check (nightly)	10m 20s
Check (1.63)	16m 16s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 35s
Test (ubuntu)	1m 20s
Test use-libc (ubuntu)	56s
Test (ubuntu-20.04)	1m 8s
Test use-libc (i686-linux)	1m 21s
Test (i686-linux)	1m 38s
Test use-libc (aarch64-linux)	1m 57s
Test (aarch64-linux)	1m 31s
Test use-libc (powerpc64le-linux)	1m 32s
Test (powerpc64le-linux)	1m 46s
Test use-libc (riscv64-linux)	1m 51s
Test (riscv64-linux)	1m 31s
Test use-libc (arm-linux)	1m 29s
Test (s390x-linux)	1m 44s
Test (arm-linux)	3m 59s
Test (ubuntu-stable)	1m 24s
Test (ubuntu-1.63)	3m 54s
Test (i686-linux-stable)	1m 35s
Test (aarch64-linux-stable)	1m 39s
Test (riscv64-linux-stable)	1m 38s
Test (s390x-linux-stable)	1m 30s
Test (powerpc64le-linux-stable)	2m 14s
Test (arm-linux-stable)	1m 39s
Test (ubuntu-1.63)	3m 50s
Test (i686-linux-1.63)	4m 20s
Test (aarch64-linux-1.63)	5m 22s
Test (riscv64-linux-1.63)	5m 21s
Test (s390x-linux-1.63)	4m 28s
Test (powerpc64le-linux-1.63)	4m 58s
Test (arm-linux-1.63)	4m 25s
Test (macos-latest)	1m 16s
Test (macos-11)	1m 17s
Test (windows)	1m 13s
Test (windows-2019)	1m 58s
	2h 1m 23s

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