[llvm-exegesis] Run unit tests on more platforms.

Summary:
 - Target-independent tests are run all the time.
 - Tests that codegen X86 code are run when X86 is in build targets.
 - Tests that run X86 jitted code are run only on X86 hosts.

Reviewers: gchatelet

Subscribers: mgorny, llvm-commits, tschuett

Differential Revision: https://reviews.llvm.org/D45614

llvm-svn: 330008