linux-user: correct handling of break exception for MIPS

Exception with break instruction has not been correctly propagated as
SIGTRAP. This resolves crash issues with examples that use break
instruction on MIPS.

Signed-off-by: Petar Jovanovic <petar.jovanovic@imgtec.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
