elf-em.h: Define and explain both EM_MIPS_RS3_LE and EM_MIPS_RS4_BE.

They have been obsoleted by the ELF header EI_CLASS and EI_DATA fields
in combination with e_flags.  Afaics EM_MIPS_RS3_LE and EM_MIPS_RS4_BE
never had any practical relevance.  Binutils will not produce such
binaries and the kernel will not accept them as MIPS binaries.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>