sys/conf: Support building kasan with gcc on arm64

[andrew]

Summary

• Remove -mllvm flags that don't exist in gcc
• Set the shadow offset with -fasan-shadow-offset

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

Sponsored by: Arm Ltd

[brooks]

I wonder if we want a CFLAGS_GEN=${CFLAGS:N-flto*:N-fno-common:N-fsanitize*:N-fno-sanitize*:N-fasan-shadow-offset*} or maybe a CFLAGS_NOSAN in kern.pre.mk so we're not having to remove arguments in so many places?

[andrew]

Rebase on D45498