[llvm] [llvm-profgen] Harden perf script invocation (PR #212253)

[Amir Ayupov via llvm-commits]

aaupov wrote:

> > Would it make more sense to modify PATH to replace perf binary lookup paths, rather than extending command line surface? BOLT uses that model, which sidesteps passing unrelated executables as perf.
> 
> @aaupov, Thank you for the good point. However, an explicit option seems preferable to modifying `PATH` because:
> 
> * It provides additional flexibility by making users independent of the `perf` binary name and other `perf`'s directory contents.
> * An invalid explicit path can be diagnosed, whereas an incorrect `PATH` override may silently fall back to the default `perf`.
> 
> Would you be open to keeping the option?

I'll defer to profgen developers what they would prefer – added reviewers

https://github.com/llvm/llvm-project/pull/212253