[libunwind] Fix build with -Wunused-function

https://reviews.llvm.org/D144252 removed -Wno-unused-function from the
libunwind build, but we have an unused function when you're building for
armv7 without assertions. Mark that function as possibly unused to avoid
the warning, and mark the parameter as a const pointer while I'm here to
make it clear that nothing is modified by a debugging function.

Reviewed By: #libunwind, philnik

Differential Revision: https://reviews.llvm.org/D156496