MIPS64: Fix wrong load HeapNumer in EmitIsMinusZero() and DoCompareMinusZeroAndBranch().

TEST: mjsunit/math-min-max
MIPS64: Fix MathExp(), MathPow() and use 2 lwus to load a heapnumber value.

TEST: mjsunit/math-exp-precision
MIPS64: Fix SubStringStub::Generate().

TEST: mjsunit/substr
MIPS64: Remove kIntAlignmentMask from class Simulator to fix build.

Define locally where needed, and rename to kInt32AlignmentMask.
MIPS64: Fix cctest.gyp to use test-assembler-mips64.cc