PR target/91441 - Turn off -fsanitize=kernel-address if TARGET_ASAN_SHADOW_OFFSET is not implemented.

 - -fsanitize=kernel-address will call targetm.asan_shadow_offset ()
   at asan_shadow_offset, so it will crash if TARGET_ASAN_SHADOW_OFFSET
   is not implemented, that's mean -fsanitize=kernel-address is not
   supported for target without TARGET_ASAN_SHADOW_OFFSET implementation.

gcc/ChangeLog:

	PR target/91441
	* toplev.c (process_options): Check TARGET_ASAN_SHADOW_OFFSET is
	implemented for -fsanitize=kernel-address, and merge check logic
	with -fsanitize=address.

testsuite/ChangeLog:

	PR target/91441
	* gcc.target/riscv/pr91441.c: New.

From-SVN: r274631