[ExecutionEngine][test][RISCV] Don't mark RISCV as unsupported (#124464)

Although MCJIT is unsupported, we can and should be running the other
tests. Stacks on top of #124463 (needed to avoid unsupported MCJIT tests
in the top-level test/ExecutionEngine directory running when they
shouldn't).

This effectively reverts b8feeba.

ninja check-llvm-executionengine before:
Total Discovered Tests: 335
  Unsupported: 335 (100.00%)

ninja check-llvm-executionengine after:
Total Discovered Tests: 335
  Unsupported      : 125 (37.31%)
  Passed           : 209 (62.39%)
  Expectedly Failed:   1 (0.30%)

llvm/test/ExecutionEngine/lit.local.cfg

@@ -1,4 +1,4 @@
-if config.root.native_target in ['Sparc', 'SystemZ', 'Hexagon', 'RISCV']:
+if config.root.native_target in ['Sparc', 'SystemZ', 'Hexagon']:
     config.unsupported = True
 
 # ExecutionEngine tests are not expected to pass in a cross-compilation setup.