MIPS: Skip embenchen tests on big-endian.

Emscripten requires little-endian arch. It has assertion for endianness:

assert(HEAPU8[0] === 255 && HEAPU8[3] === 0,
    'Typed arrays 2 must be run on a little-endian system');

TEST=mjsunit/asm/embenchen/*
BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/708663003

Patch from Paul Lind <paul.lind@imgtec.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25177 ce2b1a6d-e550-0410-aec6-3dcde31c8c00