tccgen: flex arrays etc.

Fixes potential writes past the allocated space with mostly
illegal flex array initializers. (60_errors_and_warnings.c
:test_var_array)

In exchange suspicious precautions such as section_reserve
or checks with sec->data_allocated were removed.  (There is
an hard check 'init_assert()' for now but it's meant to be
just temporary)

Also, instead of filling holes, always memset(0) structures
& arrays on stack.  Sometimes more efficient, sometimes isn't.
At least we can omit putting null initializers.

About array range inititializers:  Reparsing tokens has a
small problem with sideeffects, for example

   int c = 0, dd[] = { [0 ... 1] = ++c, [2 ... 3] = ++c };

Also, instead of 'squeeze_multi_relocs()', delete pre-existing
relocations in advance. This works even if secondary initializers
don't even have relocations, as with
    [0 ... 7] = &stuff,
    [4] = NULL

Also, in tcc.h: new macro "tcc_internal_error()"

i386-asm.c

@@ -1517,7 +1517,7 @@ ST_FUNC void subst_asm_operand(CString *add_str,
     } else if (r & VT_LVAL) {
         reg = r & VT_VALMASK;
         if (reg >= VT_CONST)
-            tcc_error("internal compiler error");
+            tcc_internal_error("");
         snprintf(buf, sizeof(buf), "(%%%s)",
 #ifdef TCC_TARGET_X86_64
                  get_tok_str(TOK_ASM_rax + reg, NULL)
@@ -1530,7 +1530,7 @@ ST_FUNC void subst_asm_operand(CString *add_str,
         /* register case */
         reg = r & VT_VALMASK;
         if (reg >= VT_CONST)
-            tcc_error("internal compiler error");
+            tcc_internal_error("");
 
         /* choose register operand size */
         if ((sv->type.t & VT_BTYPE) == VT_BYTE ||