[AArch64] Make Read Write System Registers Read Only

This patch makes the following System Registers Read Only:

 - CurrentEL
 - ICH_MISR_EL2
 - PMBIDR_EL1
 - PMSIDR_EL1

as found in:
https://developer.arm.com/docs/ddi0595/e/aarch64-system-registers

Relative line numbers were also added to the tests so we get more
informative error messages on failure.

Change-Id: I963b4f01ca5737b58f9e8e7abe9ca1d99e328758