Add constant for riscv64 architecture in Platform class

[HannesWell]

As @merks pointed out, we should add a riscv64 constant in the Platform class, that that this architecture is now supported on Linux.

Part of https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/issues/2310 @yuzibo FYI

[github-actions[bot]]

Test Results

 1 758 files  ±0   1 758 suites  ±0   1h 26m 50s :stopwatch: - 13m 53s  4 170 tests ±0   4 148 :white_check_mark: ±0   22 :zzz: ±0  0 :x: ±0  13 107 runs  ±0  12 943 :white_check_mark: ±0  164 :zzz: ±0  0 :x: ±0 

Results for commit 999f708f. ± Comparison against base commit c25f2b7c.

[yuzibo]

Thanks!