Shrink size of platform tag enum #857

Job	Run time
macos-x86_64	15m 40s
macos-aarch64	15m 22s
sdist	9m 37s
linux (x86_64-unknown-linux-gnu)	7m 50s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	11m 19s
linux (i686-unknown-linux-gnu)	11m 6s
windows (x86_64-pc-windows-msvc, x64)	14m 4s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	10m 28s
windows (i686-pc-windows-msvc, x86)	14m 16s
linux-arm (arm-unknown-linux-musleabihf, arm)	10m 42s
linux-s390x (s390x-unknown-linux-gnu, s390x)	14m 50s
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	11m 11s
musllinux (x86_64-unknown-linux-musl)	10m 17s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	11m 30s
musllinux (i686-unknown-linux-musl)	9m 55s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	11m 14s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	10m 24s
	3h 19m 45s

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