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

third_party/afl/src/docs/INSTALL

Index: third_party/afl/src/docs/INSTALL
diff --git a/third_party/afl/src/docs/INSTALL b/third_party/afl/src/docs/INSTALL
index fa8a34c357480ae4bda13e8995ea8e95217c5733..4168e88f587a6086f6d0e9fcdf33c0e152d55dfd 100644
--- a/third_party/afl/src/docs/INSTALL
+++ b/third_party/afl/src/docs/INSTALL
@@ -164,8 +164,20 @@ 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.