Get rid of mangle map stuff

Summary: This diff eliminates the need for using 'objdump' and making a map of
mangled C++ symbol names during the build. ext_hhvm was changed to instead
generate extern declarations for all the native functions defined in the
.idl files, and to use function pointer casts instead of using
"asm(<mangled-name>)" declarations.

Measurements on my dev machine show that this diff decreases hhvm build
times by 5% - 9%.

Reviewed By: @ptarjan

Differential Revision: D1556958