Add gcovr support to get code coverage for multiple configurations

[rhempel]

Currently the output of gcov is unique for each configuration of umm_malloc - for example the results of UMM_BEST_FIT are separate from UMM_FIRST_FIT. This can lead to incomplete code coverage reports.
This can be addressed by generating coverage reports for multiple configurations, and then using gcovr to combine them to get a better picture of coverage