cmd/compile: Set call flags on RISC-V call instructions

This prevents calls from being misidentified as dead code and deleted
from a load with no natural exit.  Fixes hang in test/235.go.

Change-Id: I008d96dde915b5fdf41d30311dd9bab6511d502a
Reviewed-on: https://review.gerrithub.io/352810


Reviewed-by: Michael Pratt <michael@pratt.im>