Add FuseSoC core files

[flaviens]

See #14

[dpetrisko]

Looks good to me! How do I test this out?

[flaviens]

From an empty directory you can type for example:

fusesoc library add hardfloatfork https://github.com/flaviens/HardFloat
fusesoc run --target=lint_compareRecFN hardfloat_riscv

and you should get:

INFO: Preparing ::hardfloat_riscv:0-r1                                                     
INFO: Setting up project                                                                   
                                                                                           
INFO: Building simulation model                                                            
verilator -f hardfloat_riscv_0-r1.vc -Wwarn-lint -Wwarn-style -Wno-WIDTH -Wno-DECLFILENAME 
INFO: Running                                                                              

This test is not super thorough though. The best, I think, would be to move the tests from Makefiles to FuseSoC in the future, which would simplify maintenance and leave less surface for mistakes.