kbuild: do not use -O3

Apparently, various versions of gcc have O3-related miscompiles. Looking
at the difference between -O2 and -O3 for gcc 11 doesn't indicate
miscompiles, but the difference also doesn't seem so significant for
performance that it's worth risking.

Link: https://lore.kernel.org/lkml/CAHk-=wjuoGyxDhAF8SsrTkN0-YfCx7E6jUN3ikC_tn2AKWTTsA@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CAHmME9otB5Wwxp7H8bR_i2uH2esEMvoBMC8uEXBMH9p0q1s6Bw@mail.gmail.com/
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Author: zx2c4

src/Kbuild

@@ -2,10 +2,9 @@
 #
 # Copyright (C) 2015-2019 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
 
-ccflags-y := -O3
-ccflags-$(CONFIG_WIREGUARD_DEBUG) += -DDEBUG -g
-ccflags-y += -D'pr_fmt(fmt)=KBUILD_MODNAME ": " fmt'
+ccflags-y := -D'pr_fmt(fmt)=KBUILD_MODNAME ": " fmt'
 ccflags-y += -Wframe-larger-than=2048
+ccflags-$(CONFIG_WIREGUARD_DEBUG) += -DDEBUG -g
 ccflags-$(if $(WIREGUARD_VERSION),y,) += -D'WIREGUARD_VERSION="$(WIREGUARD_VERSION)"'
 
 wireguard-y := main.o noise.o device.o peer.o timers.o queueing.o send.o receive.o socket.o peerlookup.o allowedips.o ratelimiter.o cookie.o netlink.o