[Driver][FreeBSD] Simplify ARM handling

Since FreeBSD 8 / 9 support was dropped from the Driver there is room to simplify
things with the ARM handling.

The exception model handling function can be removed.

EABI is now the default.

Reviewed By: dim

Differential Revision: https://reviews.llvm.org/D144823

(cherry picked from commit 13a10e7e)