Bug 1043144 - Don't write machc bytecode file; r=mshal

When writing bytecode, Python will append "c" to the loaded filename to
produce a bytecode file. Since "mach" was being imported, this resulted
in the creation of a "machc" file.

The implementation of imp.load_module() in CPython's import.c checks
sys.dont_write_bytecode. So, we wrap imp.load_module to set this flag
when importing mach.

--HG--
extra : rebase_source : 248a2349663affee3920a0726e10818d57c6ff17
extra : amend_source : 221280da9963cf91975658144ff3011353852fee