x86: Avoid fastcall when building with pcc
Apparently, PCC doesn't support the fastcall calling convention.
Nor does it issue a warning or error for the attribute that it
does not understand.
x86: Rewrite ffi_call
Decouple the assembly from FFI_TYPE_*. Merge prep_args with ffi_call,
passing the frame and the stack to the assembly.
Note that this patch isn't really standalone, as this breaks closures.