Improve lit "native" feature to check if the native backend is builtin

The goal is to have 'ninja check' passing even if the X86 backend is
not built.

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243958 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed