[llvm-exegesis] Minor changes for downstream consumers

This patch adjust the runConfiguration function to allow passing a
parameter to force returning SnippetCrashes as errors rather than
serializing them as strings in the returned Benchmark object. In
addition, this patch adds a getter for the signal information address in
the SnippetCrash object.

These are both needed to allow for downstream consumption of the
exegesis library within Gematria.