ArmVirtPkg/ArmVirtQemu: enable initial ID map at early boot

ArmVirtPkg/ArmVirtQemu: enable initial ID map at early boot

Now that we have all the pieces in place, switch the AArch64 version of
ArmVirtQemu to a mode where the first thing it does out of reset is
enable a preliminary ID map that covers the NOR flash and sufficient
DRAM to create the UEFI page tables as usual.

The advantage of this is that no manipulation of memory occurs any
longer before the MMU is enabled, which removes the need for explicit
coherency management, which is cumbersome and bad for performance.

It also means we no longer need to build all components that may execute
with the MMU off (including BASE libraries) with strict alignment.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>