For platforms that use function descriptors (currently AIX and
PPC64BE), log an external callback's entrypoint address rather than
its function descriptor address.  This allows proper lookup in the
tick processor's symbol table.

R=jkummerow@chromium.org, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1409993006

Cr-Commit-Position: refs/heads/master@{#31633}