This commit adds hs_try_putmvar_with_value to rts. It allows more
flexibility than hs_try_putmvar by taking an additional value argument
as a closure to be put into the MVar. This function is used & tested
by the wasm backend runtime, though it makes sense to expose it as a
public facing RTS API function as well.

(cherry picked from commit f75e823e)
(cherry picked from commit e8afc767)