Add special case handling of linux target triples that do not contain `-gnu`.

For targets that end it `redhat-linux` and `suse-linux` manually add the `-gnu`
section of the target since `linux-gnu` is needed in the testsuite.

This patch also moves the removal of minor and patchlevel numbers from OSX
triples to be handled when deducing the triple instead of when adding available
features.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220724 91177308-0d34-0410-b5e6-96231b3b80d8