Limit regress libexecinfo to amd64 or llvm >=12.

Limit regress libexecinfo to amd64 or llvm >=12.

Only amd64 has unwind info by default, so this regress test only works
on amd64. In llvm12, more architectures have unwind info by default, so
this test should work on those architectures.

Found by bluhm. OK bluhm@