MIPS/LD: Add OpenBSD/mips64 support
    
    Complement commit 26eebcf553d7 ("Update OpenBSD/mips64 support"),
    <https://sourceware.org/ml/binutils/2005-04/msg00382.html>, which added
    OpenBSD/mips64 support to GAS, and also add it to LD, avoiding a build
    failure at the configuration stage, like:
    
    *** ld does not support target mips64-unknown-openbsd
    *** see ld/configure.tgt for supported targets
    make[1]: *** [configure-ld] Error 1
    
    As per OS support only include n64 MIPS emulations, and use the
    traditional ones, matching the choice already made with the addition of
    GAS support.
    
    	ld/
    	* configure.tgt <mips64el-*-openbsd*, mips64-*-openbsd*>: New
    	targets.