Roll AFL 2.31b:2.38b and switch coverage to use trace-pc-guard.

third_party/afl/src/docs/INSTALL

 =========================
 Installation instructions
 =========================
 
   This document provides basic installation instructions and discusses known
   issues for a variety of platforms. See README for the general instruction
   manual.
 
 1) Linux on x86
 ---------------
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 You're on your own. On POSIX-compliant systems, you may be able to compile and
 run the fuzzer; and the LLVM mode may offer a way to instrument non-x86 code.
 
 The fuzzer will not run on Windows. It will also not work under Cygwin. It
 could be ported to the latter platform fairly easily, but it's a pretty bad
 idea, because Cygwin is extremely slow. It makes much more sense to use
 VirtualBox or so to run a hardware-accelerated Linux VM; it will run around
 20x faster or so. If you have a *really* compelling use case for Cygwin, let
 me know.
 
-Although Android on x86 should theoretically work, the stock kernel has SHM
-support compiled out, so you will need to address this issue first. It's
-possible that all you need is this:
+Although Android on x86 should theoretically work, the stock kernel may have
+SHM support compiled out, and if so, you may have to address that issue first.
+It's possible that all you need is this workaround:
 
   https://github.com/pelya/android-shmem
+
+Joshua J. Drake notes that the Android linker adds a shim that automatically
+intercepts SIGSEGV and related signals. To fix this issue and be able to see
+crashes, you need to put this at the beginning of the fuzzed program:
+
+  signal(SIGILL, SIG_DFL);
+  signal(SIGABRT, SIG_DFL);
+  signal(SIGBUS, SIG_DFL);
+  signal(SIGFPE, SIG_DFL);
+  signal(SIGSEGV, SIG_DFL);
+
+You may need to #include <signal.h> first.