sys/conf: Support building kasan with gcc on arm64

 - Remove -mllvm flags that don't exist in gcc
 - Set the shadow offset with -fasan-shadow-offset
 - Remove this when kasan is disabled as it's an error to use this
   when not building for kasan under gcc

This allows the arm64 GENERIC-KASAN kernel to build with gcc13.

Reviewed by:	brooks
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D45300

sys/conf/kern.pre.mk

@@ -95,13 +95,15 @@ COMPAT_FREEBSD32_ENABLED!= grep COMPAT_FREEBSD32 opt_global.h || true ; echo
 KASAN_ENABLED!=	grep KASAN opt_global.h || true ; echo
 .if !empty(KASAN_ENABLED)
 SAN_CFLAGS+=	-DSAN_NEEDS_INTERCEPTORS -DSAN_INTERCEPTOR_PREFIX=kasan \
-		-fsanitize=kernel-address \
-		-mllvm -asan-stack=true \
+		-fsanitize=kernel-address
+.if ${COMPILER_TYPE} == "clang"
+SAN_CFLAGS+=	-mllvm -asan-stack=true \
 		-mllvm -asan-instrument-dynamic-allocas=true \
 		-mllvm -asan-globals=true \
 		-mllvm -asan-use-after-scope=true \
 		-mllvm -asan-instrumentation-with-call-threshold=0 \
 		-mllvm -asan-instrument-byval=false
+.endif
 
 .if ${MACHINE_CPUARCH} == "aarch64"
 # KASAN/ARM64 TODO: -asan-mapping-offset is calculated from:
@@ -111,7 +113,11 @@ SAN_CFLAGS+=	-DSAN_NEEDS_INTERCEPTORS -DSAN_INTERCEPTOR_PREFIX=kasan \
 #	KASAN_MIN_ADDRESS, and this offset value should eventually be
 #	upstreamed similar to: https://reviews.llvm.org/D98285
 #
+.if ${COMPILER_TYPE} == "clang"
 SAN_CFLAGS+=	-mllvm -asan-mapping-offset=0xdfff208000000000
+.else
+SAN_CFLAGS+=	-fasan-shadow-offset=0xdfff208000000000
+.endif
 .elif ${MACHINE_CPUARCH} == "amd64" && \
       ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 180000
 # Work around https://github.com/llvm/llvm-project/issues/87923, which leads to