[AArch64] Use 16 bytes as preferred function alignment on Cortex-A73.

Summary:
Using 16 byte alignment is beneficial on Cortex-A73, similar to
Cortex-A72 (added in D34961).

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

Reviewed By: t.p.northover

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

Differential Revision: https://reviews.llvm.org/D35493

llvm-svn: 308283