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-compiler-rt.sh

@@ -60,7 +60,7 @@ mkdir -p "$PREFIX"
 PREFIX="$(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}}
 
 CLANG_RESOURCE_DIR="$("$PREFIX/bin/clang" --print-resource-dir)"