Driver: assume that all architectures are supported for libstc++

Rather than asserting that the target is unsupported, make a guess at what the
tree for a port would look like and use that for the search path.

Addresses review comments from Ried Kleckner for SVN r220547.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220624 91177308-0d34-0410-b5e6-96231b3b80d8