[AArch64] Use 16 bytes as preferred function alignment on Cortex-A57.

Summary:
This change gives a 0.89% speed on execution time, a 0.94% improvement
in benchmark scores and a 0.62% increase in binary size on a Cortex-A57.
These numbers are the geomean results on a wide range of benchmarks from
the test-suite, SPEC2000, SPEC2006 and a range of proprietary suites.

The software optimization guide for the Cortex-A57 recommends 16 byte
branch alignment.

Reviewers: t.p.northover, mcrosier, javed.absar, kristof.beyls, sbaranga

Reviewed By: kristof.beyls

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: https://reviews.llvm.org/D34954

llvm-svn: 307389