[trace][intelpt] Support system-wide tracing [15] - Make triple optional

The process triple should only be needed when LLDB can't identify the correct
triple on its own. Examples could be universal mach-o binaries. But in any case,
at least for most of ELF files, LLDB should be able to do the job without having
the user specify the triple manually.

Differential Revision: https://reviews.llvm.org/D126990