[AArch64] Set pref. func. align to 8 bytes on Neoverse E1 & Cortex-A65

Summary:
The Arm Neoverse E1 and Cortex-A65 Software Optimization Guide [1][2],
Section "4.7 Branch instruction alignment" state:

"It is preferable for branch targets, including subroutine entry points,
to be placed on aligned 64-bit boundaries to maximize instruction fetch
efficiency."

This patch sets the preferred function alignment on Neoverse E1 and
Cortex-A65 to 2^3=8B. This was already the case in some Cortex-A CPUs
such as Cortex-A53.

[1] https://developer.arm.com/docs/swog466751/latest/arm-neoversetm-e1-core-software-optimization-guide
[2] https://developer.arm.com/docs/swog010045/latest/arm-cortex-a65-core-software-optimization-guide

Reviewers: dmgreen, fhahn, samparker

Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65937

llvm-svn: 368431