gas: Port "copy st_size only if unset" to aarch64 and riscv

And disable the new test gas/elf/size.s for alpha which uses its own
.set, for hppa*-*-hpux* which does not allow .size before declaration.

gas/config/tc-aarch64.c

@@ -10469,17 +10469,17 @@ aarch64_elf_copy_symbol_attributes (symbolS *dest, symbolS *src)
 {
   struct elf_obj_sy *srcelf = symbol_get_obj (src);
   struct elf_obj_sy *destelf = symbol_get_obj (dest);
+  /* If size is unset, copy size from src.  Because we don't track whether
+     .size has been used, we can't differentiate .size dest, 0 from the case
+     where dest's size is unset.  */
+  if (!destelf->size && S_GET_SIZE (dest) == 0)
+    {
       if (srcelf->size)
 	{
-      if (destelf->size == NULL)
 	  destelf->size = XNEW (expressionS);
 	  *destelf->size = *srcelf->size;
 	}
-  else
-    {
-      free (destelf->size);
-      destelf->size = NULL;
-    }
       S_SET_SIZE (dest, S_GET_SIZE (src));
     }
+}
 #endif

gas/config/tc-riscv.c

@@ -4540,20 +4540,19 @@ riscv_elf_copy_symbol_attributes (symbolS *dest, symbolS *src)
 {
   struct elf_obj_sy *srcelf = symbol_get_obj (src);
   struct elf_obj_sy *destelf = symbol_get_obj (dest);
+  /* If size is unset, copy size from src.  Because we don't track whether
+     .size has been used, we can't differentiate .size dest, 0 from the case
+     where dest's size is unset.  */
+  if (!destelf->size && S_GET_SIZE (dest) == 0)
+    {
       if (srcelf->size)
 	{
-      if (destelf->size == NULL)
 	  destelf->size = XNEW (expressionS);
 	  *destelf->size = *srcelf->size;
 	}
-  else
-    {
-      if (destelf->size != NULL)
-	free (destelf->size);
-      destelf->size = NULL;
-    }
       S_SET_SIZE (dest, S_GET_SIZE (src));
     }
+}
 
 /* RISC-V pseudo-ops table.  */
 static const pseudo_typeS riscv_pseudo_table[] =

gas/testsuite/gas/elf/size.d

@@ -1,5 +1,7 @@
 #readelf: -sW
 #name: ELF symbol size
+#notarget: alpha-*-* hppa*-*-hpux*
+# The Alpha target uses its own .set pseudo-insn.
 
 #...
  +[0-9]+: 0+ +1 +NOTYPE +LOCAL +DEFAULT +[0-9]+ +foo1