e.g. -target arm-none-eabi
In your prefered build system you could add this build flag.
for Makefile:
ifeq($(YCMG), 1)
    CFLAGS += -target arm-none-eabi
endif
and start the generator in vim with:
YCMGeneratConfig -M=YCMG=1 -f

Probably this could be done automatically with the fake build system?

…of extra_flags

so you would be able to import some extra stuff like test-frameworks
ycm_extra_manual_config.py:
extra_flags = ['-I./my_testframework/inc', '-I./some_other_includes']