[TargetParser] Add missing armv8l ARMv8 variant.

This change adds the missing armv8l variant as an alias of armv8 architecture.
The issue was observed with several regressions in validation on armv8l
hardware (for instance ExecutionEngine/frem.ll failed due to lack of neon fpu).

Tested with regression testsuite passed without regression on ARM and x86_64.

Patch by Yvan Roux.

Reviewers: rengolin, rogfer01, olista01, fhahn

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D41859

llvm-svn: 322098