MIPS64: Fix wrong activiation of trace-sim with high instr counts.

Change sim icount_ to int64_t to prevent rollover, which incorrectly
activated trace-sim mode, due to trace-sim-at flag being 0 when
unused.

Leave FLAG_trace_sim_at as 'int' for now, and just cast to int64_t.
My use cases have never needed a trace-sim count greater than 2
billion instuctions.

Changing FLAG_trace_sim_at to int64 would require adding new
flag types in flag-definitions, and possibly new command-line
parsing code. It's only worth doing if we actually need it.

BUG=gh-77
TEST=v8 benchmarks
MIPS64: Fix accessor calls in simulator.

TEST = accessors tests in cctest
BUG =

(cherry picked from commit 2254f7df6d2f619951ca6f9d38daf7d74eccbeb6)