suppress set but not used warnings for kernel builds

Use -Wno-unused-but-set-variable for kernel builds with clang13.

To turn this warning back on, set the following in src.conf:
    WITH_SET_BUT_NOTUSED_KERNEL_WARNINGS=

Reviewed by:	mjg, imp
Differential Revision:	https://reviews.freebsd.org/D34784

sys/conf/kern.mk

@@ -43,7 +43,11 @@ CWARNEXTRA?=	-Wno-error=tautological-compare -Wno-error=empty-body \
 CWARNEXTRA+=	-Wno-error=shift-negative-value
 CWARNEXTRA+=	-Wno-address-of-packed-member
 .if ${COMPILER_VERSION} >= 130000
-CWARNFLAGS+=	-Wno-error=unused-but-set-variable
+.if ${MK_SET_BUT_NOTUSED_KERNEL_WARNINGS} == "no"
+CWARNEXTRA+=	${NO_WUNUSED_BUT_SET_VARIABLE}
+.else
+CWARNEXTRA+=	-Wno-error=unused-but-set-variable
+.endif
 .endif
 .endif	# clang
 

sys/conf/kern.opts.mk

@@ -60,7 +60,8 @@ __DEFAULT_NO_OPTIONS = \
     INIT_ALL_ZERO \
     KERNEL_RETPOLINE \
     RATELIMIT \
-    REPRODUCIBLE_BUILD
+    REPRODUCIBLE_BUILD \
+    SET_BUT_NOTUSED_KERNEL_WARNINGS
 
 # Some options are totally broken on some architectures. We disable
 # them. If you need to enable them on an experimental basis, you