riscv: define ILLEGAL_POINTER_VALUE for 64bit

BugLink: https://bugs.launchpad.net/bugs/2086242

commit 5c17847 upstream.

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: fbe934d ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>

Author: xhackerustc

arch/riscv/Kconfig

@@ -210,6 +210,11 @@ 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 3 if 64BIT