scripts: build-all.py: set clang as REAL_CC for arm

To support LLVM for arm, set REAL_CC to clang
which overrides gcc.
All the defconfigs we care about should be in
arch/arm/configs/vendor/ so remove the pattern-based
detection logic for arm.

Change-Id: Id48890e0d7b3ee5a37fe1a6ea2ae762bdd65eeec
Signed-off-by: Rahul Shahare <rshaha@codeaurora.org>