This provides working closure support on iOS/ARM devices where
PROT_WRITE|PROT_EXEC is not permitted. The code passes basic
smoke tests, but requires further review.
This implements support for re-mapping a shared table of executable
trampolines directly in front of a writable configuration page, working
around PROT_WRITE restrictions for sandboxed applications on Apple's
iOS.
This implementation is for testing purposes; a proper allocator is still
necessary, and ARM-specific code needs to be moved out of
src/closures.c.
