attributes: Don't try to use _BitScanReverse64 on MSVC/ARM

Despite what MSDN says, this intrinsic doesn't exist for ARM,
only for ARM64.
7 jobs for master in 2 minutes and 1 second (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build-debian #4084
debian amd64

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passed build-debian-aarch64 #4088
aarch64 debian

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passed build-debian-aarch64-clang-5 #4089
aarch64 clang5 debian

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passed build-debian-static #4085
debian amd64

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passed build-macos #4090
macos

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passed build-win32 #4086
win32

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passed build-win64 #4087
win64

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