Fix target detection for Windows ARM64EC

[kobykahane]

• Treat MSVC ARM64EC as an ARM64 target.
• Don't try setting various '-march' options for ARM64 on MSVC, where the switch does not apply.

With the accompanying changes in weidai11/cryptopp#1257, this enables building CryptoPP for Windows ARM64EC with cryptopp-cmake.

[kobykahane]

CI workflow failures appear to be caused by the issue described in weidai11/cryptopp#1240 and unrelated to this PR.

[abdes]

Thanks for the contribution.