Support for user mode and arm64ec

[DavidXanatos]

Is your feature request related to a problem? Please describe.
When working on dll's which inject into an early process loading stage, like SbieDll.dll from https://github.com/sandboxie-plus/Sandboxie it is good practice not to link against anything but the ntdll.dll, and may kernel32.dll, so no standard libs and no C run time should be included as we want to avoid linking against the accompanying dll's.
Currently that project uses only C but it would be great if c++ could be used.

Describe the solution you'd like
ucxxrt adding support for user mode and ARM64EC

Describe alternatives you've considered
Grabbing the a few obj files from MSFT's own libcmt and bunch self implemented functions works for C,
see https://github.com/sandboxie-plus/Sandboxie/tree/master/Sandboxie/common/NtCRT