Dumps all of the hand-coded unwind info for gas generated. Move jump
table data into .rodata. Adjust ffi_call_unix64 to load the static
chain. Split out sse portions of ffi_closure_unix64 to
ffi_closure_unix64_sse rather than test cif->flags at runtime.
A "ffi_go_closure" is intended to be compatible with the
function descriptors used by Go, and ffi_call_go sets up
the static chain parameter for calling a Go function.
The entry points are disabled when a backend has not been
updated, much like we do for "normal" closures.
Richard Henderson