Merge arch headers into LLVM provided include directory #1069

Job	Run time
prepare	2s
linux	2h 15m 2s
macos	3h 16m 36s
linux-asserts	0s
msys2 (mingw64)	2h 6m 46s
msys2 (clang64)	1h 46m 6s
linux-test-cross-build-ffmpeg	0s
linux-cross-aarch64	2h 17m 17s
linux-cross-windows (i686, ucrt)	2h 56m 52s
linux-cross-windows (x86_64, ucrt)	3h 1m 6s
linux-cross-windows (armv7, ucrt)	2h 55m 9s
linux-cross-windows (aarch64, ucrt)	2h 54m 34s
test-compiler-rt	0s
test-ffmpeg	0s
test-openmp	0s
test-libcxx	0s
test-toolchain (x86_64)	1m 51s
test-toolchain (i686)	1m 55s
upload-nightly	0s
	23h 33m 16s

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