x86/S3: Clarify and improve the behaviour of do_suspend_lowlevel()

x86/S3: Clarify and improve the behaviour of do_suspend_lowlevel()

do_suspend_lowlevel() behaves as a function call, even when the trampoline
jumps back into the middle of it.  Discuss this property, while renaming the
far-too-generic __ret_point to s3_resume.

Optimise the calling logic for acpi_enter_sleep_state().  $3 doesn't require a
64bit write, and the function isn't variadic so doesn't need to specify zero
FPU registers in use.

In the case of an acpi_enter_sleep_state() error, we didn't actually lose
state so don't need to restore it.  Jump straight to the end.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>