llvm/cmake/config.guess: add support for e2k (Elbrus-2000)

[r-a-sattarov]

backport patch from gnu-config:

• https://lists.gnu.org/archive/html/config-patches/2015-03/msg00000.html

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[r-a-sattarov]

bug fix

• https://bugs.llvm.org/show_bug.cgi?id=42828