Make RV's lit tests portable

[simoll]

Add proper loopvectorizer tests to RV with the lit infrastructure.

The feature/port-cdl-fixes merge adds LIT testing infra for RV from the NEC RV version. There is also a check-rv target now running both functional tests as well as the lit tests. This issue is about making the lit tests more portable, that is:

• Remove the dependence on the exact build configuration of the llvm-ve-rv stack.
• Port/add tests for more likely host targets (such as x86, arm , ..). Check for compiler feature flags to turn these tests on/off.
• Add a reliable way to call the RV loopvectorizer from lit tests (RVPLUG may not be linked in, etc.).