cmm-sources are not included in shared object linking

[bgamari]

Describe the bug
cmm-sources objects are not linked into shared objects.

To Reproduce
Steps to reproduce the behavior:

$ cd Cabal/cabal-testsuite/PackageTests/CmmSources
$ cabal v2-build --enable-executable-dynamic
Resolving dependencies...
Build profile: -w ghc-8.8.3 -O1
In order, the following will be built (use -v for more details):
 - cmmexperiment-0 (exe:demo) (first run)
Preprocessing executable 'demo' for cmmexperiment-0..
Building executable 'demo' for cmmexperiment-0..
Linking /opt/exp/ghc/ghc-landing/libraries/Cabal/cabal-testsuite/PackageTests/CmmSources/dist-newstyle/build/x86_64-linux/ghc-8.8.3/cmmexperiment-0/x/demo/build/demo/demo ...
/opt/exp/ghc/ghc-landing/libraries/Cabal/cabal-testsuite/PackageTests/CmmSources/dist-newstyle/build/x86_64-linux/ghc-8.8.3/cmmexperiment-0/build/libHScmmexperiment-0-inplace-ghc8.8.3.so: error: undefined reference to 'aToMyWordzh'
collect2: error: ld returned 1 exit status
`cc' failed in phase `Linker'. (Exit code: 1)

Please use version-prefixed commands (e.g. v2-build or v1-build) to avoid ambiguity.

Expected behavior
A runnable executable should be producd.

System information

• Any
• cabal 5aea8a4

Additional context
Add any other context about the problem here.