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-openmp.sh

@@ -42,7 +42,7 @@ 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}}
 
 if [ ! -d llvm-project/openmp ] || [ -n "$SYNC" ]; then
     CHECKOUT_ONLY=1 ./build-llvm.sh