riscv: Fix asan-stack clang build

Nathan reported that because KASAN_SHADOW_OFFSET was not defined in
Kconfig, it prevents asan-stack from getting disabled with clang even
when CONFIG_KASAN_STACK is disabled: fix this by defining the
corresponding config.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Fixes: 8ad8b727 ("riscv: Add KASAN support")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>