[AArch64] Use 8 bytes as preferred function alignment on Cortex-A53.

Summary:
This change gives a 0.25% speedup on execution time, a 0.82% improvement
in benchmark scores and a 0.20% increase in binary size on a Cortex-A53.
These numbers are the geomean results on a wide range of benchmarks from
the test-suite and a range of proprietary suites.

Reviewers: t.p.northover, aadg, silviu.baranga, mcrosier, rengolin

Reviewed By: rengolin

Subscribers: grimar, davide, aemerson, rengolin, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D35568

llvm-svn: 309494