acpica: Quiet a -Wdangling-pointer warning in AcpiUtInitStackPtrTrace.

This function intentionally saves a pointer to an on-stack variable in
a global as a dubious way of reading the stack pointer.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D37535

(cherry picked from commit f4432625edd64df94ce7c2a085f256dbc4d2b215)