Update global variable definitions. No functional change.

Move all of the public globals to acpixf.h for the convenience
of users. Also:

Adds #ifndef/#endif conditions arround ACPI_GLOBAL and
ACPI_INIT_GLOBAL definition so that OSPMs might be able to:
1. Redefine ACPI_GLOBAL/ACPI_INIT_GLOBAL into no-op, and
2. Redefine external global variables into immediates to implement stubs
   for them.
Lv Zheng.