MIPS/LD: Add OpenBSD/mips64 support

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.