[gwp_asan] Use __sanitizer_fast_backtrace on Fuchsia

[cryptoad]

Adding @hctim for visibility.

compiler-rt/lib/gwp_asan/optional/backtrace_fuchsia.cpp

16–22

I am not sure I understand the sentence (might be my English skills), should this be provides?

[hctim]

LGTM with Kostya's nit. Glad to see that the modularity works for Fuchsia out of the box :).

[phosek]

LGTM

[mcgrathr]

compiler-rt/lib/gwp_asan/optional/backtrace_fuchsia.cpp

16–22

Yes, it was just a typo. Thanks.

[mcgrathr]

typo fix

compiler-rt/lib/gwp_asan/optional/backtrace_fuchsia.cpp

 //===-- backtrace_fuchsia.cpp -----------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "gwp_asan/optional/backtrace.h"
 
-// GWP-ASan on Fuchsia doesn't currently support backtraces.
+#include <zircon/sanitizer.h>
 
 namespace gwp_asan {
 namespace backtrace {
 
-options::Backtrace_t getBacktraceFunction() { return nullptr; }
+// Fuchsia's C library provides safe, fast, best-effort backtraces itself.
+options::Backtrace_t getBacktraceFunction() {
+  return __sanitizer_fast_backtrace;
+}
+
+// These are only used in fatal signal handling, which is not used on Fuchsia.
+
 PrintBacktrace_t getPrintBacktraceFunction() { return nullptr; }
 SegvBacktrace_t getSegvBacktraceFunction() { return nullptr; }
 
 } // namespace backtrace
 } // namespace gwp_asan