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 <[email protected]>
Signed-off-by: Miodrag Dinic <[email protected]>
Signed-off-by: Aleksandar Markovic <[email protected]>
Acked-by: Alex Bennée <[email protected]>
Tested-by: Emilio G. Cota <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>