Refactor the x86 CPU name logic in the driver and pass -march and -mcpu
flag information down from the Clang driver into the Gold linker plugin
for LTO. This allows specifying -march on the linker commandline and
should hopefully have it pass all the way through to the LTO optimizer.

Fixes PR14697.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172354 91177308-0d34-0410-b5e6-96231b3b80d8