The x86_64 unix port only handles one ABI; don't define all of the
other symbols. The UNIX64 symbol retains the same value.
The i386 ports ought to have the same symbols, even if we can't yet
unify the values without incrementing the libffi soname.
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