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When AP1 memory is being emulated (long descriptor page tables are in
use), the AbortTrap machinery is used to emulate usermode read access.
This provides coverage for all read instructions except those that
AbortTrap handles via MemMap requests - LDREX, LDA, LDAEX, LDF & LFM.

LDREX & LDAEX request both read & write access, so are fine (the MemMap
request will get passed through to the registered AbortTrap handlers).

LDF & LFM are irrelevant, since they only exist on ARM7500FE (on other
machines FPEmulator will translate them to regular LDR/LDM, which are
handled correctly)

LDA however, will generate a plain "memmap with usermode read" request.
When AbortTrap looks at the permissions of emulated AP1 it doesn't take
into account the fact that the usermode read permission is being
emulated, so it thinks that everything is fine and claims the memmap
was successful, causing the abort handler to retry the instruction
without making any changes, resulting in an infinite abort loop.

Deal with this by detecting the above situation and also requesting
usermode execute access. This will avoid the kernel (and hopefully the
registered AbortTrap handlers) from thinking that the emulated AP1 is
acceptable, without adversely affecting the behaviour of other
instructions or access privileges. If no handler is present or the
memmap request is denied, the abort will get passed on to the next stage
of the abort handler (i.e. you'll get a standard data abort from trying
to LDA from arbitrary emulated AP1 memory)

The new test program (Dev/AbortTrap/attest_ap1) will check that this
edge case is dealt with correctly.

Tested on Pi 4, for both long & short page tables

Version 6.59. Tagged as 'Kernel-6_59'