The prefix ERR_WARN_ is unnecessarily long and may be a disincentive
to create new warning categories. Change it to WARN_*, it is still
plenty distinctive.
This is equivalent to nasm-2.14.xx checkin 77f53ba6d4cb90e5a7e09b33357ed7c1fe9f6b9d.
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>