Recommit: "[llvm-exegesis] Improve error reporting in Target.cpp"

Summary: Commit 14191596 was reverted in
abe01e17 because it broke builds testing
without libpfm. A preparatory commit <commit_sha1> was added to enable
this recommit.

Original commit message:

Followup to D74085.
Replace the use of `report_fatal_error()` with returning the error to
`llvm-exegesis.cpp` and handling it there.

Differential Revision: https://reviews.llvm.org/D74113