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.toolchain

@@ -16,7 +16,7 @@ WORKDIR /build
 
 ENV TOOLCHAIN_PREFIX=/opt/llvm-mingw
 
-ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
+ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64 arm64ec"
 
 # Copy prebuilt toolchains for the current architecture.
 # Test executing a binary, to make sure that it works (i.e. it is not built