Update constant value for exnref type

[sbc100]

We are currently using -0x18, but that slot just used by reference-types for nullref:
WebAssembly/reference-types#66

[aheejin]

I think that -18 in WebAssembly/reference-types#66 is in decimal, while our -0x18 is in hexadecimal.

[sbc100]

Oh, that case wabt is just wrong I guess: https://github.com/WebAssembly/wabt/blob/76ff3af40040a90c15b6b9f2614f3a3869dbdab1/src/common.h#L217

I'll propose a change there.

[aheejin]

Not sure what you mean. The current wabt implementation looks correct. Exnref is -0x18, in hexadecimal.