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.

build-libcxx.sh

@@ -72,12 +72,6 @@ else
     esac
 fi
 
-for arch in $ARCHS; do
-    case $arch in
-    arm64ec) ARM64X_FLAGS="-marm64x" ;;
-    esac
-done
-
 for arch in $ARCHS; do
     [ -z "$CLEAN" ] || rm -rf build-$arch
     mkdir -p build-$arch
@@ -85,20 +79,12 @@ for arch in $ARCHS; do
     [ -n "$NO_RECONF" ] || rm -rf CMake*
 
     EXTRA_CFLAGS=""
-    EXTRA_LDFLAGS=""
     case $arch in
     i686|x86_64)
         # Force using the mingw stdio functions, for correct long double
         # printing.
         EXTRA_CFLAGS="-D__USE_MINGW_ANSI_STDIO=1"
         ;;
-    aarch64)
-        EXTRA_CFLAGS="$ARM64X_FLAGS"
-        EXTRA_LDFLAGS="$ARM64X_FLAGS"
-        ;;
-    arm64ec)
-        continue
-        ;;
     esac
 
     cmake \
@@ -133,8 +119,6 @@ for arch in $ARCHS; do
         -DLIBCXXABI_LIBDIR_SUFFIX="" \
         -DCMAKE_C_FLAGS_INIT="$CFGUARD_CFLAGS $EXTRA_CFLAGS" \
         -DCMAKE_CXX_FLAGS_INIT="$CFGUARD_CFLAGS $EXTRA_CFLAGS" \
-        -DCMAKE_ASM_FLAGS_INIT="$EXTRA_CFLAGS" \
-        -DCMAKE_SHARED_LINKER_FLAGS="$EXTRA_LDFLAGS" \
         ..
 
     cmake --build . ${CORES:+-j${CORES}}