[AArch64] Sets the preferred function alignment for Cortex-A53/A55.

Setting the preffered function alignment to 16 for Cortex A53/A55
improves performance in a wide range of benchmarks. This brings it
in line with the Cortex-A53/A55 tuning that is used in GCC
(gcc/config/aarch64/aarch64.c).

Differential Revision: https://reviews.llvm.org/D101636

Change-Id: I2ce47fe7ab5e3b54f49c89038d8da4e404742de2