PR target/96260 - KASAN should work even back-end not porting anything.

 - Most KASAN function don't need any porting anything in back-end
   except asan stack protection.

 - However kernel will given shadow offset when enable asan stack
   protection, so eveything in KASAN can work if shadow offset is given.

 - Verified with x86 and risc-v.

 - Verified with RISC-V linux kernel.

gcc/ChangeLog:

	PR target/96260
	* asan.c (asan_shadow_offset_set_p): New.
	* asan.h (asan_shadow_offset_set_p): Ditto.
	* toplev.c (process_options): Allow -fsanitize=kernel-address
	even TARGET_ASAN_SHADOW_OFFSET not implemented, only check when
	asan stack protection is enabled.

gcc/testsuite/ChangeLog:

	PR target/96260
	* gcc.target/riscv/pr91441.c: Update warning message.
	* gcc.target/riscv/pr96260.c: New.