tccasm: synch C and asm symtab tighter

tccasm: synch C and asm symtab tighter

See testcase.  The C and asm symtab are still separate,
but integrated tighter: the asm labels are only synched at file
end, not after each asm snippet (this fixes references from one
to another asm block), the C and asm syms are synched both ways,
so defining things in asm and refering from C, or the other way
around works.  In effect this model reflects what happens with
GCC better.

For this the asm labels aren't using the C label namespace anymore,
but their own, which increases the size of each TokenSym by a pointer.