Use default CLANG build options for ARM

We previously disabled CLANG_FULL on (little-endian) ARM because the
build failed. This is no longer the case and as of Clang 3.5 we cannot
build any part of the in-tree Clang with in-tree GCC, so it's no longer
necessary to disable CLANG_FULL.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D2525