KVM: selftests: arm64: Add GCS EXLOCK exception emulation test

When running in vEL2 ERET is emulated, this means that we must emulate
the behaviour of GCSCR_EL2.EXLOCKEN. Add a test that validates the
behaviour.

As per DDI0487 R_WTXBY PSTATE.EXLOCK is set to the value of
GCSCR_ELx.EXLOCKEN when taking an exception to the same exception level.
As per DDI0487 R_TYTWB if on exception return to the same EL
GCSCR_ELx.EXLOCKEN is 1 and PSTATE.EXLOCK is not 1 then the exception
return is illegal.

We nest handling of two UDFs to allow us to both observe and modify
PSTATE.EXLOCK, checking that expected exceptions are generated for ERET.
We also repeat the UDF tests having enabled fine grained traps for ERET
to try to ensure that the slow paths are taken.

We also attempt to write to both SPSR_ELx and ELR_ELx, again using a UDF
handler to control PSTATE.EXLOCK. We do all this both with and without
having enabled EXLOCKEN, though only for code running at EL2 due to
framework limitations.

For completeness we also inject a data abort from EL0 to EL2 with
GCSCR_EL2.EXLOCKEN set, ensuring that PSTATE.EXLOCK is not set.

Signed-off-by: Mark Brown <broonie@kernel.org>