Remove Triple Vendor check when creating Linux Platform

Summary: The Linux Platform shouldn't care about the Vendor field in the Triple. Currently it allows a value of "PC", or "unknown" if LLDB was built on Linux. This patch removes that check, so the Vendor field isn't touched. This will allow the Linux Platform to be created when using a Triple of *-*-Linux.

Reviewers: vharron, clayborg, sas, tberghammer

Reviewed By: clayborg, sas, tberghammer

Subscribers: tberghammer, sas, lldb-commits

Differential Revision: http://reviews.llvm.org/D8742

llvm-svn: 237052