ACPI/sleep: Convert acpi_wakeup_address into a function

Convert acpi_wakeup_address from a raw variable into a function so that
x86 can wrap its dereference of the real mode boot header in a function
instead of broadcasting it to the world via a #define.  This sets the
stage for a future patch to move the definition of acpi_wakeup_address()
out of asm/acpi.h and thus break acpi.h's dependency on asm/realmode.h.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191119002121.4107-12-sean.j.christopherson@intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>