Revert "* and #pragma pop_macro("macro_name")"

- pop_macro incorrect with initially undefined macro
- horrible implementation (tcc_open_bf)
- crashes eventually (abuse of Sym->prev_tok)

- the (unrelated) asm_label part is the opposite of a fix
  (Despite of its name this variable has nothing to do with
  the built-in assembler)

This reverts commit 0c8447db79.

tccgen.c

@@ -152,6 +152,7 @@ static inline Sym *sym_malloc(void)
 ST_INLN void sym_free(Sym *sym)
 {
     sym->next = sym_free_first;
+    tcc_free(sym->asm_label);
     sym_free_first = sym;
 }