Since Linux 4.1, flag -f became -F, but the long option --flags stayed
the same. Using --flags assures compatibility with new and old versions.

Also, symbols appear to be reported as name+offset in newer versions
even when not requested, so we trim the latter part if present.

BUG=v8:4899
LOG=N

Review-Url: https://codereview.chromium.org/2134703002
Cr-Commit-Position: refs/heads/master@{#37614}