Skip core file tests on build configurations lacking necessary components

Summary:
To successfully open a core file, we need to have LLVM built with
support for the relevant target. Right now, if one does not have the
appropriate targets configured, the tests will fail.

This patch uses the GetBuildConfiguration SB API to inform the test (and
anyone else who cares) about the list of supported LLVM targets. The
test then uses this information to approriately skip the tests.

Reviewers: clayborg, jingham

Subscribers: martong, lldb-commits

Differential Revision: https://reviews.llvm.org/D48641

llvm-svn: 335859