riscv: define ILLEGAL_POINTER_VALUE for 64bit

This is used in poison.h for poison pointer offset. Based on current
SV39, SV48 and SV57 vm layout, 0xdead000000000000 is a proper value
that is not mappable, this can avoid potentially turning an oops to
an expolit.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Fixes: fbe934d6 ("RISC-V: Build Infrastructure")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240705170210.3236-1-jszhang@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>