KVM: arm64: selftests: Add MMIO sign-extending load test

KVM: arm64: selftests: Add MMIO sign-extending load test

Add a test for sign-extending MMIO loads (LDRSB, LDRSH, LDRSW) into Xt
and Wt destinations, with and without the sign bit set. The host supplies
the MMIO data and checks the guest register holds the sign-extended value.

Repeat the loads big-endian on a mixed-endian implementation. Issue those
at EL0: SCTLR_EL1.EE would make an EL1 load big-endian but also walk the
little-endian page tables big-endian, whereas SCTLR_EL1.E0E selects only
EL0 data endianness and leaves the walk little-endian.

Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20260706115522.954913-3-fuad.tabba@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>