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.

run-tests.sh

@@ -27,7 +27,7 @@ export PATH=$PREFIX/bin:$PATH
 : ${CORES:=$(nproc 2>/dev/null)}
 : ${CORES:=$(sysctl -n hw.ncpu 2>/dev/null)}
 : ${CORES:=4}
-: ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}
+: ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64 arm64ec}}
 
 MAKE=make
 if command -v gmake >/dev/null; then