rts: add __wrapped_freeJSVal

This commit wraps imported freeJSVal in a __wrapped_freeJSVal C
function for wasm backend RTS. In general, wasm imports are only
supposed to be directly called by C; they shouldn't be used as
function pointers, which confuses wasm-ld at link-time when generating
shared libraries.

(cherry picked from commit 33d9db17)