Let's not rely on explicit HHVM version numbers in xdebug tests

Summary: We bump the version number quite a bit. Let's not hardcode them in the xdebug tests

Reviewed By: @ptarjan

Differential Revision: D1534339
Clean up printing functions & some const usage in vasm

Summary: Rename some printing functions to be more consistent with the rest of
the jit, make them work with const references, and move function declarations
to vasm-print.h to match the definitions in vasm-print.cpp.

Reviewed By: @edwinsmith

Differential Revision: D1532792