For platforms that use function descriptors (currently AIX and
PPC64BE), log an external callback's entrypoint address rather than
its function descriptor address.

R=jkummerow@chromium.org, michael_dawson@ca.ibm.com
TEST=cctest/test-cpu-profiler/JsNativeJsSample
BUG=

Review URL: https://codereview.chromium.org/1752173003

Cr-Commit-Position: refs/heads/master@{#34505}