build: support compiling .mm on host

libchrome uses .mm (Objective-C++) files to bridge C++ code with
OS X Frameworks. This adds support for compiling .mm to .o by just
using the existing C++ support.

Bug: 24168923
Change-Id: Ia65357e2e2584dfffcb6796e214fe6b27635c3a6
2 files changed