Add Additional Flags to PGO Build

Add Additional Flags to PGO Build

In GCC, we can add a couple more flags to give us confidence that the profile
data is actually being found and used.

Also, my system for example doesn't have a binary installed under the name
`llvm-profdata`, but it does have, e.g., `llvm-profdata-13`, etc. So this
commit adds a variable that can be overridden.