MIPS: Add big-endian support for MIPS.

Important notices:

- The snapshot cannot be created for big-endian target in cross-compilation
  environment on little-endian host using simulator.

- In order to have i18n support working on big-endian target, the icudt46b.dat and
  icudt46b_dat.S files should be generated and upstreamed to ICU repo.

- The mjsunit 'nans' test is endian dependent, it is skipped for mips target.

TEST=
BUG=

Conflicts:

	Makefile
	test/cctest/test-platform.cc
	test/mozilla/mozilla.status
	test/test262/test262.status
	tools/testrunner/local/statusfile.py