Fix gcc color diagnostics

gcc color diagnostics stopped working when the flag was moved to
TARGET_GLOBAL_CFLAGS, as that gets overwritten by combo/select.mk.  Put
it back in COMMON_GLOBAL_CFLAGS, and then let the windows build filter
it out, similar to the way clang deals with unknown flags.

Change-Id: I2db221edb893d81f199494e7515d1b9282c12fae