tccasm: Implement .set sym, expr

That, as well as "sym = expr", if expr contains symbols.
Slightly tricky because a definition from .set is overridable,
whereas proper definitions aren't.

This doesn't yet allow using this for override tricks from C
and global asm blocks because the symbol tables from C and asm
are separate.

tcctok.h

@@ -308,6 +308,7 @@
  DEF_ASMDIR(align)
  DEF_ASMDIR(balign)
  DEF_ASMDIR(p2align)
+ DEF_ASMDIR(set)
  DEF_ASMDIR(skip)
  DEF_ASMDIR(space)
  DEF_ASMDIR(string)