Compile libunwind in release mode (#102)

Default build is `DEBUG`. Switch to `RelWithDebInfo`.

```
**CMAKE_BUILD_TYPE**:STRING
  Sets the build type for ``make`` based generators. Possible values are
  Release, Debug, RelWithDebInfo and MinSizeRel. On systems like Visual Studio
  the user sets the build type with the IDE settings.
```

Makefile

@@ -150,6 +150,7 @@ $(LIBDIR)/libunwind.a:
 			-DLLVM_ENABLE_RUNTIMES="libunwind" \
 			-DLIBUNWIND_ENABLE_SHARED=OFF -DLIBUNWIND_ENABLE_STATIC=ON \
 			-DLIBUNWIND_INCLUDE_DOCS=OFF \
+			-DCMAKE_BUILD_TYPE=RelWithDebInfo \
 			-DCMAKE_INSTALL_PREFIX="$(CURDIR)/install/usr" \
 			$(CMAKE_ARGS) build $(HANDLE_OUTPUT) && \
 		$(MAKE) -C build VERBOSE=$(V) clean $(HANDLE_OUTPUT) && \