MSVC Compat: Move stdatomic fallback to its own directory

Mirrors what we do for GCC.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
9 jobs for master in 3 minutes and 3 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed build-debian #228098
amd64 debian

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passed build-debian-aarch64 #228102
debian aarch64

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passed build-debian-aarch64-clang-5 #228103
debian clang5 aarch64

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passed build-debian-static #228099
amd64 debian

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passed build-debian-werror #228105
debian aarch64

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passed build-macos #228104
macos

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passed build-win32 #228100
win32

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passed build-win64 #228101
win64

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  Test
passed test-debian #228106
amd64 debian

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