RTS linker maps code as writable

GHC's RTS linker maps executable code in writable pages, representing a significant potential exploit point for arbitrary code execution. OpenBSD disallows running program that do this by default.

Instead we should first map pages as PROT_READ | PROT_WRITE, perform any necessary relocations (which requires writing), and then mprotect it to PROT_READ | PROT_EXEC.

To find the relevant code grep for PROT_EXEC in the rts/ directory.