MIPS/BFD: Remove EI_ABIVERSION 5 allocation for PT_GNU_STACK support
    
    Revert commit 17733f5be961 ("Increment the ABIVERSION to 5 for MIPS
    objects with non-executable stacks.") and remove EI_ABIVERSION 5
    allocation for PT_GNU_STACK support, which has not made it to glibc
    and will be reassigned.
    
    	bfd/
    	* bfd/elfxx-mips.c (_bfd_mips_post_process_headers): Revert
    	2016-02-23 change and remove EI_ABIVERSION 5 support.
    
    (cherry picked from commit 4d4f40e041ce7c8c7c8e50f957e0440b64ab7e95)