MIPS: Add big-endian support for MIPS.

Important notices:

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

- In order to have i18n support working on big-endain 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.

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