Do not use #warning to warn about missing implementation of dt_popc(),

but just have a comment that this is broken.

This is just a bandaid until somebody can fix this correctly.  The code
is just a broken as it was before r223262 - now buildworld just doesn't
fail.

Tested by:	i386 + amd64 buildworld
With hat:	benl co-mentor

cddl/contrib/opensolaris/lib/libdtrace/common/dt_subr.c

@@ -828,7 +828,7 @@ dt_popc(ulong_t x)
 	x = x + (x >> 32);
 	return (x & 0x7F);
 #else
-# warning need td_popc() implementation
+/* This should be a #warning but for now ignore error. Err: "need td_popc() implementation" */
 #endif
 }