build.rs: fix mips32r6 detection for libc feature switch #3311

Job	Run time
Rustfmt	10s
Check --no-default-features (nightly)	1m 12s
Check nightly-only targets (nightly)	34s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 30s
Check selected Tier 3 platforms (nightly)	4m 23s
Check (stable)	9m 5s
Check (nightly)	12m 21s
Test (ubuntu)	1m 23s
Check (1.63)	16m 18s
Test use-libc (ubuntu)	1m 28s
Test (ubuntu-20.04)	1m 7s
Test use-libc (i686-linux)	1m 35s
Test (i686-linux)	1m 42s
Test use-libc (aarch64-linux)	1m 25s
Test (aarch64-linux)	1m 32s
Test use-libc (powerpc64le-linux)	1m 29s
Test (powerpc64le-linux)	1m 43s
Test use-libc (riscv64-linux)	1m 34s
Test (riscv64-linux)	1m 27s
Test use-libc (arm-linux)	1m 43s
Test (s390x-linux)	1m 32s
Test (arm-linux)	2m 1s
Test (ubuntu-stable)	1m 11s
Test (i686-linux-stable)	1m 23s
Test (aarch64-linux-stable)	1m 39s
Test (riscv64-linux-stable)	1m 19s
Test (s390x-linux-stable)	1m 35s
Test (powerpc64le-linux-stable)	1m 37s
Test (arm-linux-stable)	1m 53s
Test (ubuntu-1.63)	4m 21s
Test (i686-linux-1.63)	4m 42s
Test (aarch64-linux-1.63)	4m 56s
Test (riscv64-linux-1.63)	5m 12s
Test (s390x-linux-1.63)	5m 56s
Test (powerpc64le-linux-1.63)	5m 33s
Test (arm-linux-1.63)	4m 47s
Test (macos-latest)	1m 41s
Test (macos-11)	1m 10s
Test (windows)	1m 36s
Test (windows-2019)	1m 14s
	1h 58m 59s

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