Compensate for src/common/lib/libc/atomic/atomic_init_cas.c losing
the extern declaration of __libc_atomic_init() and instead using a
new one added to src/lib/libc/include/extern.h

That file is outside src/common and src/sys so is unavailable to
kernel builds, so instead make a new include file in src/common
which the kernel can read which contains the needed extern decl
for __libc_atomic_init()

This seems to fix the evbarm builds (maybe others) - but it is
all MUCH TOO UGLY to keep. Someone please make a better fix,
even if that just means reverting rev 1.5 of
src/common/lib/libc/atomic/atomic_init_cas.c
and all of the changes here (the addition to libc/include/extern.h
should be harmless to keep).