[X86] Make elfiamcu an OS, not an environment.

GNU tools require elfiamcu to take up the entire OS field, so, e.g.
i?86-*-linux-elfiamcu is not considered a legal triple.
Make us compatible.

Differential Revision: http://reviews.llvm.org/D14081

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251390 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed