powerpc/powernv: move OPAL call wrapper tracing and interrupt handling to C

The OPAL call wrapper gets interrupt disabling wrong. It disables
interrupts just by clearing MSR[EE], which has two problems:

- It doesn't call into the IRQ tracing subsystem, which means tracing
  across OPAL calls does not always notice IRQs have been disabled.

- It doesn't go through the IRQ soft-mask code, which causes a minor
  bug. MSR[EE] can not be restored by saving the MSR then clearing
  MSR[EE], because a racing interrupt while soft-masked could clear
  MSR[EE] between the two steps. This can cause MSR[EE] to be
  incorrectly enabled when the OPAL call returns. Fortunately that
  should only result in another masked interrupt being taken to
  disable MSR[EE] again, but it's a bit sloppy.

The existing code also saves MSR to PACA, which is not re-entrant if
there is a nested OPAL call from different MSR contexts, which can
happen these days with SRESET interrupts on bare metal.

To fix these issues, move the tracing and IRQ handling code to C, and
call into asm just for the low level call when everything is ready to
go. Save the MSR on stack rather than PACA.

Performance cost is kept to a minimum with a few optimisations:

- The endian switch upon return is combined with the MSR restore,
  which avoids an expensive context synchronizing operation for LE
  kernels. This makes up for the additional mtmsrd to enable
  interrupts with local_irq_enable().

- blr is now used to return from the opal_* functions that are called
  as C functions, to avoid link stack corruption. This requires a
  skiboot fix as well to keep the call stack balanced.

A NULL call is more costly after this, (410ns->430ns on POWER9), but
OPAL calls are generally not performance critical at this scale.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>