MIPS64: Squash of previous commits.

MIPS64: Fixes Math.pow() error for edge cases.

MathPow Stub expected signed extended input.
The test fails for pow(+Infinity,-1).

TEST=test262/S15.8.2.13_A12

MIPS64: Add missing instructions for conversion to mips64 simulator.

TEST=

MIPS64: Fixed cctests for assembler and simulator.

Fixed ADDU, ADDIU, SUBU and LUI to sign-extend result.
Fixed warnings on compiling cctest.
Fixed style issues.

TEST=cctest/test-assembler-mips64.cc/MIPS2, MIPS11, MIPS16

MIPS64: Removed changed if() for checking kArchVariant.

As MIPS64 port is separated physically from MIPS port, check is irrelevant now, as MIPS64 executable will be run on MIPS64 hardware.

TEST=

Conflicts:
	src/mips64/simulator-mips64.cc