[GWP-ASan] Stub out backtrace/signal functions on Fuchsia

The initial version of GWP-ASan on Fuchsia doesn't support crash and
signal handlers, so this just adds empty stubs to be able to compile
the project on the platform.

Differential Revision: https://reviews.llvm.org/D90537

GitOrigin-RevId: 98fe39df939ee6f1fd20b75edcc9fe0860b52dba
2 files changed