MIPS: Environment for testing v8 built for big-endian target on little-endian host.

This approach uses qemu user mode for cross-compilation and testing v8 for big-endian
target on little endian host.

To use qemu for cross-compilation which includes snapshot creation on LE host:

- Build qemu from source from current trunk and put the 'qemu-mips' in PATH.

  Get the source:  git clone git://git.qemu-project.org/qemu.git

- Set path to target libaries and dynamic linker:

  export QEMU_LD_PREFIX="path_to_cross_toolchain/mips-linux-gnu/libc/"

- Set up cross-compilation environment

  export CC,CXX,AR,RANLIB,LINK varibles for cross-comiplation

- Buld v8 for mips:

  make mips.release -j8

To run v8 tests under qemu, pass --qemu flag to tools/run-test.py script.

For example:

tools/run-tests.py --arch=mips --mode=release  --timeout=480 --verbose
   --time --no-presubmit --report cctest  -j8 --qemu | tee cctest.log

Running tests under qemu is supported for other architectures than mips.
It is required to have appropriate qemu in path, and set up QEMU_LD_PREFIX
env. variable.