[OpenEmbedded] Add OpenEmbedded vendor

Summary: The lib paths are not correctly picked up for OpenEmbedded sysroots
(like arm-oe-linux-gnueabi). I fix this in a follow-up clang patch. But in
order to add the correct libs I need to detect if the vendor is oe. For this
reason, it is first necessary to teach llvm to detect oe vendor, which is what
this patch does.

Reviewers: chandlerc, compnerd, rengolin, javed.absar

Reviewed By: compnerd

Subscribers: kristof.beyls, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D48861

llvm-svn: 336401