Enable the ARM64 target on the ARM64EC ABI.

[degasus]

ARM64EC enables both _M_AMD64 and _M_ARM64EC. We already exclude ARM64EC from the SSE2 detection, but it was missing for the X64 target detection (by me in #1152, apparently on purpose).

However we're now fine to enable the ARM64 target. At least my kernels compile fine (as good as on the native arm64) on MSVC.

Related: https://techcommunity.microsoft.com/blog/windowsosplatform/getting-to-know-arm64ec-defines-and-intrinsic-functions/2957235

Tested on a local Windows arm64 machine with the arm64ec ABI:
now:

[doctest] doctest version is "2.4.9"
[doctest] run with "--help" for options
===============================================================================
[doctest] test cases:  406 |  406 passed | 0 failed | 0 skipped
[doctest] assertions: 5999 | 5999 passed | 0 failed |
[doctest] Status: SUCCESS!

before enabling XSIMD_TARGET_ARM64:

[doctest] doctest version is "2.4.9"
[doctest] run with "--help" for options
===============================================================================
[doctest] test cases:  94 |  94 passed | 0 failed | 0 skipped
[doctest] assertions: 803 | 803 passed | 0 failed |
[doctest] Status: SUCCESS!

[serge-sans-paille]

LGTM. This does mean we should add that architecture to our CI target, can you give it a try?

[degasus]

LGTM. This does mean we should add that architecture to our CI target, can you give it a try?

Done. CMake's recormendation seems to be to provide /arm64EC as envvar, else you need to set also linking flags. But we overwrite the linker flags, I had to fix this as well.

The strangest issue I had with the test is the git's msys2 envvar handling: When calling cmake (native) from bash (msys2), it converts /arm64EC to C:/Program files/GIT/arm64EC... A single space works around this issue now...