build-mingw-w64: Split out building tools to a separate script

This simplifies build-mingw-w64.sh. When building on native
windows (where we also provide unprefixed tools, we can't have
the yet incomplete compiler toolchain in $PATH when building
the mingw-w64 tools. (Logically, build-mingw-w64-tools belong
with build-llvm and install-wrappers; using the original system
compiler to build tools that are supposed to run on the current
machine, as opposed to the cross target, like when building the
runtimes.)

Dockerfile.dev

@@ -42,6 +42,9 @@ ARG DEFAULT_CRT=ucrt
 COPY build-mingw-w64.sh ./
 RUN ./build-mingw-w64.sh $TOOLCHAIN_PREFIX --with-default-msvcrt=$DEFAULT_CRT
 
+COPY build-mingw-w64-tools.sh ./
+RUN ./build-mingw-w64-tools.sh $TOOLCHAIN_PREFIX
+
 # Build compiler-rt
 COPY build-compiler-rt.sh ./
 RUN ./build-compiler-rt.sh $TOOLCHAIN_PREFIX