RISC-V: Interpreter intrinsify Thread.currentThread()

Calling intrinsic version of Thread.currentThread() in interpreter is ~30% faster on the Unmatched board:
Before:
Benchmark Mode Cnt Score Error Units
MyBenchmark.testCurrentThread avgt 5 4665.765 ± 212.906 ns/op
After:
Benchmark Mode Cnt Score Error Units
MyBenchmark.testCurrentThread avgt 5 3381.415 ± 223.005 ns/op