MIPS64: Fix ExternalCallbackScope() to adapt 64-bit simulator.

Fixed in r19311/75295c56

TEST: cctest/test-cpu-profiler/NativeAccessorUninitializedIC (only fix on sim)
      cctest/test-cpu-profiler/NativeAccessorMonomorphicIC (only fix on sim)
      cctest/test-cpu-profiler/NativeMethodMonomorphicIC (only fix on sim)
Merge remote-tracking branch 'mrb/yy-dev-mips64-2' into pldev-05
MIPS64: Add operands for smi untagging in macro-assembler.
MIPS64: Add LoadSmiUntag(), LoadSmiUntagToIndex(), SmiToIndex()

Intent is to allow Loading, Untagging, and Shifting/Scaling to
be combined. This should allow common source for mips32 & mips64
for these operations.

Note that the optimizations around these are different for mips32
and mips64, and the intent is to minimize total instructions on
both platforms.

Includes one example usage from Dusan's prior CL.
MIPS64: Fix TODO smi ops to use LoadSmiUntag and friends.