ACPI/x86: revert 'x86, acpi: Call acpi_enter_sleep_state via an asmlinkage C function from assembler'

cd74257b
patched up GTS/BFS -- a feature we want to remove.
So revert it (by hand, due to conflict in sleep.h)
to prepare for GTS/BFS removal.
Signed-off-by: NLen Brown <len.brown@intel.com>
Acked-by: NIngo Molnar <mingo@kernel.org>
Acked-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>