Fixes#7593
By default MacOS concretizes using the clang compiler. The unit tests
include a call to "spack spec mpileaks", which has elfutils as a
dependency; #7096 added a conflict in elfutils to avoid building
with clang, which lead to the MacOS unit tests to start failing.
This updates the concretization to force using gcc when concretizing
mpileaks.
5e758b979e