KVM: arm64: nv: Work around lack of pauth support in old toolchains

We still support GCC 8.x, and it appears that this toolchain
usually comes with an assembler that does not understand "pauth"
as a valid architectural extension.

This results in the NV ERETAx code breaking the build, as it relies
on this extention to make use of the PACGA instruction (required
by assemblers such as LLVM's).

Work around it by hand-assembling the instruction, which removes the
requirement for any assembler directive.

Fixes: 6ccc971ee2c6 ("KVM: arm64: nv: Add emulation for ERETAx instructions")
Reported-by: Linaro Kernel Functional Testing <lkft@linaro.org>
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>

KVM: arm64: nv: Add emulation for ERETAx instructions

FEAT_NV has the interesting property of relying on ERET being
trapped. An added complexity is that it also traps ERETAA and
ERETAB, meaning that the Pointer Authentication aspect of these
instruction must be emulated.

Add an emulation of Pointer Authentication, limited to ERETAx
(always using SP_EL2 as the modifier and ELR_EL2 as the pointer),
using the Generic Authentication instructions.

The emulation, however small, is placed in its own compilation
unit so that it can be avoided if the configuration doesn't
include it (or the toolchan in not up to the task).

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240419102935.1935571-13-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>