build-busybox: Disable make, copy mingw32-make.exe as make.exe

The built-in make is a POSIX make tool, which doesn't support
GNU make features.

As we do have GNU make, insert that as "make.exe" in the busybox
directory. Don't do that in the main llvm-mingw bin directory,
which can be used with other environments (e.g. msys2), where the
native make is to be preferred.

build-busybox.sh

@@ -74,6 +74,7 @@ make mingw64a_defconfig O=$BUILDDIR -j$CORES
 cd $BUILDDIR
 sed -ri 's/^(CONFIG_AR)=y/\1=n/' .config
 sed -ri 's/^(CONFIG_FEATURE_FAIL_IF_UTF8_MANIFEST_UNSUPPORTED)=y/\1=n/' .config
+sed -ri 's/^(CONFIG_MAKE)=y/\1=n/' .config
 $MAKE -j$CORES CROSS_COMPILE=${HOST+$HOST-}
 cp ../LICENSE $PREFIX/LICENSE.txt
 mkdir -p $PREFIX/bin