target/mips: Prepare loads/stores for EVA

target/mips: Prepare loads/stores for EVA

EVA load and store instructions access the user mode address map, so
they need to use mem_idx of MIPS_HFLAG_UM. Update the various utility
functions to allow mem_idx to be more easily overridden from the
decoding logic.

Specifically we add a mem_idx argument to the op_ld/st_* helpers used
for atomics, and a mem_idx local variable to gen_ld(), gen_st(), and
gen_st_cond().

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Yongbok Kim <yongbok.kim@imgtec.com>
Cc: Yongbok Kim <yongbok.kim@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>