Fix subscreen credits Zelda workaround

The second part of the credit cheat were executed independenly of the first
part. This was caused but the wrong syntax used for the if statement in commit
8e14cc9ba55560492ad2d0e3a077769032c58d8f ("Add Missing Subscreen Delay Fix for
Ocarina of Time")
Fix interpreter marks for unused JR_IDLE and JALR_IDLE

The JR_IDLE and JALR_IDLE functions are unused in the pure interpreter and
cached interpreter. Thus these functions are marked with the attribute
"unused". Unfortunatelly, the declaration did not precede the definition. Also
the definition conflicted with the declaration.
Provide definition of __GNUC_PREREQ for OpenBSD

OpenBSD doesn't seem to have the definition for __GNUC_PREREQ and thus fails to
i386 with PIC enabled (which is the default now).
Merge pull request #3 from ecsv/llvm

Fix LLVM warnings and OpenBSD build failures