[lld] [lld] [MinGW] Support targeting ARM64EC (PR #78911)

[Billy Laws via llvm-commits]

bylaws wrote:

@mstorsjo I think even with what MinGW is doing `arm64ecpe` makes more sense over `aarch64ecpe`, for the same reason that `__arm64ec__` is defined and not `__aarch64ec__`

https://github.com/llvm/llvm-project/pull/78911