Extend the aarch64-w64-mingw32 prefix to provide ARM64EC libraries
alongside standard AArch64 libraries, replacing the separate
arm64ec-w64-mingw32 prefix. This aligns with how MSVC distributes
ARM64EC libraries and enables the linker to process both targets
simultaneously into a single hybrid binary.
This extensively utilizes LLVM's -marm64x option, compiling both
targets in a single compiler invocation to produce hybrid object
files (unlike MSVC tools, which require separate compilation passes).
If ARCHS itself was set by the caller, it would already have been
implicitly passed to the makefile - but if the caller set
TOOLCHAIN_ARCHS instead, that wasn't acted upon by the makefile.
As the shell script already has the logic for initializing this
(in the same fashion as all other shell scripts), just use that
and pass the value on to the makefile.
Jacek Caban