Disable libLTO tests when libLTO is not built

Summary:
The current test only checks whether ld64 is available, causing tests
to fail when ld64 is avilable but libLTO is not built.

Reviewers: beanz, mehdi_amini

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D27739

llvm-svn: 289662