Fix ComputeSupportExeDirectory for Linux (merge with Android).

Summary:
ComputeSupportExeDirectory relied on ComputeSharedLibraryDirectory which was
not always reliable. Using procfs seems to be the best way to deal with it on
Linux (since it's already done on Android, might as well merge it).

Reviewers: ovyalov

Reviewed By: ovyalov

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8511

llvm-svn: 232883