Include arm64ec in the lists of all architectures

Since LLVM 21.x, the upstream support for ARM64EC in LLD and
compiler-rt is complete enough for us to be able to build it.

build-mingw-w64.sh

@@ -87,7 +87,7 @@ unset CC
 : ${CORES:=$(nproc 2>/dev/null)}
 : ${CORES:=$(sysctl -n hw.ncpu 2>/dev/null)}
 : ${CORES:=4}
-: ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}
+: ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64 arm64ec}}
 
 if [ -z "$SKIP_INCLUDE_TRIPLET_PREFIX" ]; then
     HEADER_ROOT="$PREFIX/generic-w64-mingw32"