Use goma when USE_GOMA=true is set

With USE_GOMA, the path to gomacc in $GOMA_DIR or $HOME/goma
will be appended to CC_WRAPPER and CXX_WRAPPER.

Note this works only with USE_NINJA. Unlike ninja, GNU make
cannot change the parallelism depending on targets. Specifying
-j500 to GNU make would mean you may run 500 local jobs in
parallel, but with -j32 goma will just slow down the build.

Change-Id: I0f571454fd2a5b525ee29b445f7ab8715927ca00