[libFuzzer] Add --dump-input-on-failure to help diagnose AArch64 failures

The fork-siguser.test and fork.test intermittently fail on the AArch64
buildbot. Unfortunately these failures are not reproducible on a similar
machine and seem to fail when the machines are under load. Before
suggesting the tests be marked unsupported for AArch64 we'd like to see
if we can get some more information about the failures to see if it helps
us reproduce. This patch adds --dump-input-on-failure to the FileCheck
commands to see if we can get some more information about the failures.

Differential Revision: https://reviews.llvm.org/D61315

llvm-svn: 359675