target/riscv: From tick(), set ebreak* if necessary.

This involves halting the target, which might have unintended side
effects, but when the debugger is connected software breakpoints must
trap to the debugger. Anything else is a terrible user experience.

Change-Id: I1f7bb610eeeb054cc3042dc6bcfc16589ce12a31
Signed-off-by: Tim Newsome <tim@sifive.com>