bsd.own.mk also adds it for gcc only (so we end up with 2 -marm for gcc).
Just add -marm unconditionally in bsd.own.mk in ARM_APCS_FLAGS and remove
it from everywhere else.
2. gcc-8 now warns about thumb internetwork in APCS mode. This does not make
sense because the documentation says that this flag does not make a
difference in apcs mode, but might produce larger code in non-APCS mode.
So we just add it to silence the warnings.
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