Fix sub-int returns on x86-64 and i386

Fix sub-int returns on x86-64 and i386

the ABIs (and other compilers) extend sub-int return values in the
caller.  TCC extends them in the callee.  For compatibility with
those other compilers we have extend them in the caller as well.
That introduces a useless double extension in pure TCC-compiled code,
but fixing that generally requires that the code generator of TCC would
understand sub-int types.  For the time being bite the bullet.