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.

test-libcxx-module.sh

@@ -24,7 +24,7 @@ PREFIX="$1"
 PREFIX="$(cd "$PREFIX" && pwd)"
 export PATH=$PREFIX/bin:$PATH
 
-: ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}
+: ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64 arm64ec}}
 
 case $(uname) in
 MINGW*|MSYS*|CYGWIN*)