Fix unused parameter warning for mips builds

libbacktrace_offline was inheriting -Wno-unused-parameter from llvm's
makefiles.  Fix an unused parameter warning when compiling for an
unsupported architecture, and return false since *value wasn't
written.

Test: lunch aosp_mips-eng && mma -j
Change-Id: I7df990f01e1025ca17ec3717107df56d2349f3cb