Implement clang-target-wrapper as an executable for windows

This allows non-msys build systems to use the triplet prefixed
compiler frontends on windows.

Keep the old shell script version as well, for easier hackability
on unix platforms.

wrappers/clang-target-wrapper.sh

@@ -9,6 +9,7 @@ if [ -n "$CCACHE" ]; then
     CCACHE=ccache
 fi
 
+# If changing this wrapper, change clang-target-wrapper.c accordingly.
 CLANG="$DIR/clang"
 FLAGS=""
 case $EXE in