riscv: Disable KSAN_SANITIZE for vDSO

riscv: Disable KSAN_SANITIZE for vDSO

We use the generic C VDSO implementations of a handful of clock-related
functions.  When kasan is enabled this results in asan stub calls that
are unlikely to be resolved by userspace, this just disables KASAN
when building the VDSO.

Verified the fix on a kernel with KASAN enabled using vDSO selftests.

Link: https://lore.kernel.org/lkml/CACT4Y+ZNJBnkKHXUf=tm_yuowvZvHwN=0rmJ=7J+xFd+9r_6pQ@mail.gmail.com/
Tested-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
[Palmer: commit text]
Fixes: ad5d1122b82f ("riscv: use vDSO common flow to reduce the latency of the time-related functions")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>