[Builtins] Use 4 byte alignment for __aeabi_memclr.

Summary:
Align __aeabi_memclr to 4 bytes. All other ARM functions are already aligned to
4-bytes in compiler-rt.
(Split off from review D38227)

Reviewers: compnerd, peter.smith, srhines, weimingz, rengolin, kristof.beyls

Reviewed By: compnerd

Subscribers: aemerson, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D38271

llvm-svn: 314255