[Dexter] Ignore step information in __libc_start_call_main

The test dexter/feature_tests/commands/perfect/expect_step_kind/direction.cpp
was failing on a machine because __libc_start_call_main was not identified as a
"frame below main" (a frame we don't want to gather information from), causing
dexter to count one more step than expected in the test.

Add __libc_start_call_main to the list of "frames below main". There may
be a more robust way of doing this but this is a pragmatic solution we can
use for now.