Fixes issue with gdb segfaulting when reading debug symbols

Summary: Changing the gcc debug optimization level to -gO (enables some optimizations
that do not interfere with debugging). gdb no longer segfaults.
Closes https://github.com/facebook/hhvm/pull/4450

Reviewed By: @sgolemon

Differential Revision: D1742603

Signature: t1:1742603:1419577719:b486deec56efeb9650a71dd55a2b82a95c1aa25f