reproduce tool: measure gdb loading time

[fuzzah]

Heavy binaries (e.g. 500+ megs) may timeout while reproducing due to long loading time in gdb.

[fuzzah]

Note to myself: we can reuse one gdb process, with the heavy app already loaded. Just don't restart gdb, clear terminal, and run reproduce cmds again for the next bug sample.
This would drastically improve bb-reproduce's performance.