aarch64: Add support for (M)ADDPT and (M)SUBPT instructions

aarch64: Add support for (M)ADDPT and (M)SUBPT instructions

The following instructions are added in this patch:

 - ADDPT and SUBPT - Add/Subtract checked pointer
 - MADDPT and MSUBPT - Multiply Add/Subtract checked pointer

These instructions are part of Checked Pointer Arithmetic extension.
This patch adds assembler and disassembler support for these instructions
with relevant checks. Tests are included as well.

A new flag "+cpa" added to documentation. This flag enables CPA extension.

Regression tested on the aarch64-none-linux-gnu target and no regressions
have been found.