[ARM] builtins: Replace abort by assert in clear_cache.

Summary:
__builtion___clear_cache maps to clear_cache function. On Linux,
clear_cache functions makes a syscall and does an abort if syscall fails.
Replace the abort by an assert so that non-debug builds do not abort
if the syscall fails.

Fixes PR34588.

Reviewers: rengolin, compnerd, srhines, peter.smith, joerg

Reviewed By: rengolin

Subscribers: aemerson, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D37788

llvm-svn: 314322