[Clang][AArch64] Define x86_64 macros for ARM64EC targets (#65420)

The ARM64EC ABI requires that struct layouts match between regular
x86_64 code and ARM64EC code. Ensure this is always the case by defining
the same set of macros as are set when targeting x86_64 but with the
addition of `__arm64ec__/_M_ARM64EC` macros that can be used for any
ARM64EC specific code.

More details can be found here:
https://techcommunity.microsoft.com/t5/windows-os-platform-blog/getting-to-know-arm64ec-defines-and-intrinsic-functions/ba-p/2957235