MIPS: Add big-endian support for MIPS, including crankshaft and hard-float.

Current issue summary for BE:

- The snapshot has to be turned off for cross-compilation to BE targets.
- All tests pass, but with ICU (i18n) support turned off, it appears there is
  an endianness related bug in ICU extension.
- The mjsunit 'nans' test is endian dependent, and should be rewritten.
- It is not tested for Soft-float ABI.