i386: use __fixdfdi instead of __tcc_cvt_ftol

Variants __fixsfdi/__fixxfdi are not needed for now because
the value is converted to double always.

Also:
- remove __tcc_fpinit for unix as it seems redundant by the
  __setfpucw call in the startup code
- avoid reference to s->runtime_main in cross compilers
- configure: fix --with-libgcc help
- tcctok.h: cleanup

tccpe.c

@@ -1800,7 +1800,9 @@ static void pe_add_runtime(TCCState *s1, struct pe_info *pe)
 
     if (TCC_OUTPUT_MEMORY == s1->output_type) {
         pe_type = PE_RUN;
+#ifdef TCC_IS_NATIVE
         s1->runtime_main = start_symbol;
+#endif
     } else {
         pe->start_addr = (DWORD)tcc_get_symbol_err(s1, start_symbol);
     }