Identify arm64ec as aarch64 instead of x64

[sivileri]

ARM64EC is a new build target for Windows ARM64 devices for x64 support.
Currently that build flavor fails in Blake3 due to attempting to use the x64 intrinsics instead of the aarch64 ones.

Please see this related change with a possible way to fix it.

[oconnor663]

Thanks for the report, we should definitely fix this. The second line changed in that diff makes sense to me, but I'm surprised by this first line:

#if (defined(__x86_64__) || defined(_M_X64)) && !defined(_M_ARM64EC)

Why is that needed? Is _M_X64 defined when _M_ARM64EC is defined?

[elichai]

Why is that needed? Is _M_X64 defined when _M_ARM64EC is defined?

According to: https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-170:

_M_X64 Defined as the integer literal value 100 for compilations that target x64 processors or ARM64EC. Otherwise, undefined.