Summary:
We do this for vanilla way already.
Let's also merge sections for profiling way and use it instead of the .a
library when it exists.

Test Plan:
```
 $ inplace/bin/ghc-stage2 --interactive -prof -fexternal-interpreter
 GHCi, version 8.7.20180921: http://www.haskell.org/ghc/  :? for help
 Prelude> pid <- System.Posix.Process.getProcessID
 Prelude> maps <- readFile $ "/proc/" ++ show pid ++ "/maps"
 Prelude> pid
 3807346
 Prelude> putStrLn $ unlines $ take 20 $ lines maps
 00400000-02103000 r-xp 00000000 00:1a 199277344
 /data/users/watashi/ghc/inplace/lib/bin/ghc-iserv-prof
 02104000-02106000 r--p 01d03000 00:1a 199277344
 /data/users/watashi/ghc/inplace/lib/bin/ghc-iserv-prof
 02106000-02417000 rw-p 01d05000 00:1a 199277344
 /data/users/watashi/ghc/inplace/lib/bin/ghc-iserv-prof
 02417000-0280a000 rw-p 00000000 00:00 0
 [heap]
 40098000-400b0000 rwxp 000d2000 00:1a 199276023
 /data/users/watashi/ghc/libraries/bytestring/dist-install/build/HSbytestring-0.10.8.2.p_o
 400b7000-400d8000 rwxp 00000000 00:00 0
 401d1000-401d2000 rwxp 000e9000 00:1a 199276023
 /data/users/watashi/ghc/libraries/bytestring/dist-install/build/HSbytestring-0.10.8.2.p_o
 40415000-40419000 rwxp 0000b000 00:1a 199275165
 /data/users/watashi/ghc/libraries/deepseq/dist-install/build/HSdeepseq-1.4.4.0.p_o
 404f8000-40526000 rwxp 000af000 00:1a 199274234
 /data/users/watashi/ghc/libraries/ghc-prim/dist-install/build/HSghc-prim-0.5.3.p_o
```

Reviewers: simonmar, bgamari, austin, hvr

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5169