Add a hard-coded FFI_EXEC_TRAMPOLINE_TABLE arm implementation.

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.
2 files changed