call AcpiLeaveSleepStatePrep after re-enabling interrupts

I want to do this change because this call (actually,
AcpiHwLegacyWakePrep) does a memory allocation and ACPI namespace
evaluation. Although it is not very likely to run into any trouble, it
is still not safe to make those calls with interrupts disabled.
witness(4) and malloc(9) do not currently check for a context with
interrupts disabled via intr_disable and we lack a facility for doing
that. So, those unsafe operations fly under the radar. But if
intr_disable in acpi_EnterSleepState was replaced with spinlock_enter
(which it probably should be), then witness and malloc would immediately
complain.

Also, AcpiLeaveSleepStatePrep is documented as called when interrupts
are enabled. It used to require disabled interrupts, but that
requirement was changed a long time ago when support for _BFS and _GTS
was removed from ACPICA.

The ACPI wakeup sequence is very sensitive to changes. I consider this
change to be correct, but there can be fallouts from it.

What AcpiHwLegacyWakePrep essentially does is writing a value
corresponding to S0 into SLP_TYPx bits of PM1 Control Register(s).
According to ACPI specifications that write should be a NOP as SLP_EN
bit is not set. But I see in some chipset specifications that they
allow to ignore SLP_EN altogether and to act on a change of SLP_TYPx
alone.

Also, there are a couple of accesses to ACPI hardware before the new
location of the call to AcpiLeaveSleepStatePrep. One is to clear the
power button status and the other is to enable SCI. So, the move may
affect the interaction between then OS and ACPI platform.

I have not seen any regressions on my test system, but it's a desktop.

MFC after: 5 weeks