Enable nimbus to be built on RISC-V computers

[haurog]

To enable nimbus to be built on risc-v boards specific ISA instruction sets and extensions need to be specified. Some tests have shown the 'gc' extension to be necessary to build nimbus on a Lichee Pi 4a and a HiFive Unmatched board. The 'gc' extensions also seems to be the minimal requirement to run a more complex Linux distribution so I expect them to be available on all boards which will be able to run nimbus.

On the Lichee Pi 4a Nimbus could sync up and follow the holesky testnet chain with these settings. Due to disk space limitations the backfill could not complete.

[github-actions[bot]]

Unit Test Results

         9 files  ±0    1 334 suites  ±0   30m 10s :stopwatch: +5s   4 999 tests ±0    4 651 :heavy_check_mark: ±0  348 :zzz: ±0  0 :x: ±0  20 877 runs  ±0  20 473 :heavy_check_mark: ±0  404 :zzz: ±0  0 :x: ±0 

Results for commit c022d956. ± Comparison against base commit b5c0b479.