Force the linker to keep symbols in lowmem on MacOSX

Summary: We can't reach them from the TC with offsets
if they're way up there in 0x0000000100000000 and higher...

Closes #3642

Reviewed By: @ptarjan

Differential Revision: D1552243
Force the php5/ng opcache to on

Summary: Docs say it's on by default, this doesn't appear to be true.

Reviewed By: @JoelMarcey

Differential Revision: D1552557