x86-64: fix flags and zero-pad long doubles

This fixes a bug introduced in commit
    8d107d9ffd
that produced wrong code because of interference between
0x10 bits VT_CONST and x86_64-gen.c:TREG_MEM

Also fully zero-pad long doubles on x86-64 to avoid random
bytes in output files which disturb file comparison.

tccgen.c

@@ -709,8 +709,12 @@ ST_FUNC int gv(int rc)
             /* XXX: not portable yet */
 #if defined(__i386__) || defined(__x86_64__)
             /* Zero pad x87 tenbyte long doubles */
-            if (size == LDOUBLE_SIZE)
+            if (size == LDOUBLE_SIZE) {
                 vtop->c.tab[2] &= 0xffff;
+#if LDOUBLE_SIZE == 16
+                vtop->c.tab[3] = 0;
+#endif
+            }
 #endif
             ptr = section_ptr_add(data_section, size);
             size = size >> 2;