Add architecture specific implementation details for KFENCE and enable
KFENCE for the riscv64 architecture. In particular, this implements the
required interface in <asm/kfence.h>.

KFENCE requires that attributes for pages from its memory pool can
individually be set. Therefore, force the kfence pool to be mapped at
page granularity.

Testing this patch using the testcases in kfence_test.c and all passed.
Signed-off-by: NLiu Shixin <liushixin2@huawei.com>
Acked-by: NMarco Elver <elver@google.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NPalmer Dabbelt <palmerdabbelt@google.com>