target/mips: reimplement SC instruction emulation and use cmpxchg

target/mips: reimplement SC instruction emulation and use cmpxchg

Completely rewrite conditional stores handling. Use cmpxchg.

This eliminates need for separate implementations of SC instruction
emulation for user and system emulation.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtec.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>