arm64: fpsimd: Move fpsimd save/restore inline

arm64: fpsimd: Move fpsimd save/restore inline

Currently the FPSIMD register save/restore sequences are written in
out-of-line assembly routines. While this works, it's somewhat painful:

* As KVM needs to be able to use the sequences in hyp code, separate
  assembly files are used for the regular kernel and KVM code. While the
  common logic is shared in assembly macros, this still requires some
  duplication, and has lead to some trivial divergence.

* For historical reasons, the assembly macros take some register
  arguments as numerical indices (e.g. "fpsimd_save x0, 8" uses x0 and
  x8), which is simply confusing.

* For historical reasons, the SVE save/restore code and FPSIMD
  save/restore code have distinct sequences for FPSR and FPCR. Ideally
  this logic would be shared.

* The assembly sequences can't be instrumented, and so it's harder than
  necessary to catch memory safety issues.

To handle the above, move the FPSIMD register save/restore sequences to
inline assembly, and share the FPSR+FPCR save/restore with SVE.

Neither GCC nor LLVM instrument memory arguments to inline assembly, so
explicit instrumentation is added in the same manner as other assembly
routines. This instrumentation is implicitly disabled by Kbuild for nVHE
hyp code.

I've used the SVE sequence for restoring FPCR, which uses an
unconditional write to FPCR, rather than the conditional write used by
the FPSIMD assembly sequence. I believe that in practice, this doesn't
matter to a real workload, and given it's possible for the mis-predicted
branch to cost more than the necessary micro-architectural
synchronization, I strongly suspect any performance impact is within the
noise.

Looking at the history, the FPSIMD assembly sequence was changed to use
a conditional write to FPCR since 2014 in commit:

  5959e25729a5 ("arm64: fpsimd: avoid restoring fpcr if the contents haven't change")

... as described in the commit message, this was based on an expectation
of implementation style, and was not based on benchmarking.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: James Morse <james.morse@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oupton@kernel.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>