build-mingw-w64: Install COPYING files

Install copyright notices into the final package. This makes LLVM MinGW
comply with many of the licenses (which require attribution), and also
makes it easier for users of LLVM MinGW to audit licenses of software
they use.

build-mingw-w64.sh

@@ -124,3 +124,9 @@ for arch in $ARCHS; do
     cd ..
 done
 cd ..
+
+for arch in $ARCHS; do
+    for file in COPYING COPYING.MinGW-w64/COPYING.MinGW-w64.txt COPYING.MinGW-w64-runtime/COPYING.MinGW-w64-runtime.txt; do
+        install -Dm644 "$file" "$PREFIX/$arch-w64-mingw32/share/mingw32/$(basename "$file")"
+    done
+done