mingw-build: Place libwinpthread-1.dll in 'lib' directory

Place `libwinpthread-1.dll` in `lib` directory so that it can be
discovered using `clang -print-file-name=libwinpthread-1.dll` command
line, as done by the Zephyr SDK LLVM build script.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

mingw-build.sh

@@ -103,6 +103,12 @@ build_mingw_toolchain() {
   make install
   popd
 
+  # Place libwinpthread-1.dll in 'lib' directory so that it can be discovered
+  # using 'clang -print-file-name=libwinpthread-1.dll'.
+  pushd ${prefix}/x86_64-w64-mingw32
+  cp bin/libwinpthread-1.dll lib
+  popd
+
   # Restore environment
   popd
   export PATH=${old_path}