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

tccelf.c

@@ -178,11 +178,11 @@ LIBTCCAPI void *tcc_get_symbol(TCCState *s, const char *name)
     return (void*)(uintptr_t)get_elf_sym_addr(s, name, 0);
 }
 
-#ifdef TCC_IS_NATIVE
+#if defined TCC_IS_NATIVE || defined TCC_TARGET_PE
 /* return elf symbol value or error */
 ST_FUNC void* tcc_get_symbol_err(TCCState *s, const char *name)
 {
-    return (void*)get_elf_sym_addr(s, name, 1);
+    return (void*)(uintptr_t)get_elf_sym_addr(s, name, 1);
 }
 #endif