Fix ghc-pkg when only prof way is enabled

Summary:
We saw following errors:

```
$ cabal install --disable-library-vanilla --disable-shared --enable-library-profiling
hashable-1.2.7.0: cannot find any of
["libHShashable-1.2.7.0-Q2TKVDwk4GBEHmizb4teZ.a",
 "libHShashable-1.2.7.0-Q2TKVDwk4GBEHmizb4teZ.p_a",
 "libHShashable-1.2.7.0-Q2TKVDwk4GBEHmizb4teZ-ghc8.4.3.so",
 "libHShashable-1.2.7.0-Q2TKVDwk4GBEHmizb4teZ-ghc8.4.3.dylib",
 "HShashable-1.2.7.0-Q2TKVDwk4GBEHmizb4teZ-ghc8.4.3.dll"]
```

This is because ghc-pkg is looking for
`libHShashable-1.2.7.0-Q2TKVDwk4GBEHmizb4teZ.p_a` instead of
`libHShashable-1.2.7.0-Q2TKVDwk4GBEHmizb4teZ_p.a`.

Test Plan: ./validate

Reviewers: simonmar, bgamari

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5234