riscv: Support RANDOMIZE_KSTACK_OFFSET

Inspired from arm64's implement -- commit 70918779aec9
("arm64: entry: Enable random_kstack_offset support")

Add support of kernel stack offset randomization while handling syscall,
the offset is defaultly limited by KSTACK_OFFSET_MAX() (i.e. 10 bits).

In order to avoid trigger stack canaries (due to __builtin_alloca) and
slowing down the entry path, use __no_stack_protector attribute to
disable stack protector for do_trap_ecall_u() at the function level.

Acked-by: Palmer Dabbelt <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Song Shuai <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

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