x86/acpi: make "asmlinkage" part first thing in the function definition

g++ insists that function declaration must start with extern "C"
(which asmlinkage expands to).

gcc doesn't care.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>