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.

Dockerfile.dev

@@ -34,7 +34,7 @@ RUN ./build-lldb-mi.sh $TOOLCHAIN_PREFIX
 COPY strip-llvm.sh ./
 RUN ./strip-llvm.sh $TOOLCHAIN_PREFIX
 
-ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
+ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64 arm64ec"
 
 # Install the usual $TUPLE-clang binaries
 COPY wrappers/*.sh wrappers/*.c wrappers/*.h wrappers/*.cfg ./wrappers/