db2a6676 added `-po` option, but
the part that parses it was missing.

Test Plan:
On a simple file:
```
./inplace/bin/ghc-stage2 A.hs -prof -main-is A; ./A +RTS -P -potest
```
produced test.prof file and didn't produce A.prof file.

```
./A +RTS -P
```
produced A.prof file

Reviewers: simonmar, bgamari, austin, erikd

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3946