Revert "Add ARM64X support"

This reverts commit 64effa9a71.

It turns out that the LLD handling of the TLS data structures
("_tls_used" and "_tls_index") needs more work, causing the
arm64ec side of arm64x binaries to not get them hooked up properly.

Dockerfile.dev

@@ -148,8 +148,5 @@ RUN cd test && \
 
 RUN cd test && \
     for arch in $TOOLCHAIN_ARCHS; do \
-        case $arch in
-        arm64ec) continue ;;
-        esac
         cp $TOOLCHAIN_PREFIX/$arch-w64-mingw32/bin/*.dll $arch || exit 1; \
     done