Add support for ARM64EC when building with MSVC

[dpaoliello]

• Building for ARM64EC requires passing the -arm64EC flag to cl.exe and /machine:arm64ec to lib.exe: https://learn.microsoft.com/en-us/windows/arm/arm64ec-build#developer-command-prompt
• Updated tool discovery to handle arm64ec as a target arch by using the aarch64 paths.
• When targeting arm64ec the lib path must ALSO include the arm64ec VC libs BEFORE the aarch64 libs.
• Added a new TargetArch wrapper type to avoid splitting the target string multiple times or calling contains when we could be using string comparison.