make buildworld failed with error "relocation truncated to fit: R_ARM_JUMP24 against symbol `_fini'"

[Andrew Turner]

I've disabled setting -mlong-calls on the clang libraries for now,
however I expect we will need to enable it again when clang 3.8.0 is
imported. As such I would recommend anyone wishing to run buildworld on
arm to update before this is imported.

Andrew