make pthread attribute types structs, even when they just have one field

this change is to get the right tags for C++ ABI matching. it should
have no other effects.

src/thread/pthread_condattr_setclock.c

@@ -3,7 +3,7 @@
 int pthread_condattr_setclock(pthread_condattr_t *a, clockid_t clk)
 {
 	if (clk < 0 || clk-2U < 2) return EINVAL;
-	*a &= 0x80000000;
-	*a |= clk;
+	a->__attr &= 0x80000000;
+	a->__attr |= clk;
 	return 0;
 }