Remove -mlong-calls vhen building arm libraries and llvm.

Clang from 9.0.0 onwards already has the necessary relocation range
extenders, so this workaround is no longer needed (it produces longer
and slower code). Tested on real hardware, and in cross-compile
environment.

Submitted by: mmel

(cherry picked from commit da759cfa320d5076b075d15ff3f00ab3ba5634fd)