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 <[email protected]>
Fixes: fbe934d ("RISC-V: Build Infrastructure")
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: xhackerustc

arch/riscv/Kconfig

@@ -320,6 +320,11 @@ config GENERIC_HWEIGHT
 config FIX_EARLYCON_MEM
 	def_bool MMU
 
+config ILLEGAL_POINTER_VALUE
+	hex
+	default 0 if 32BIT
+	default 0xdead000000000000 if 64BIT
+
 config PGTABLE_LEVELS
 	int
 	default 5 if 64BIT