x86: Tidy ffi_abi

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.

src/x86/ffi.c

@@ -260,7 +260,7 @@ ffi_status ffi_prep_cif_machdep(ffi_cif *cif)
     }
 
 #ifndef X86_WIN32
-  if (cif->abi == FFI_SYSV || cif->abi == FFI_UNIX64)
+  if (cif->abi == FFI_SYSV)
     cif->bytes = (cif->bytes + 15) & ~0xF;
 #endif