[RISCV][NFC] Rename RequiredExtensions to RequiredFeatures.

The field 'RequiredExtensions' is used to specify the constraint for rvv builtin, and it contains something which is not a sub-extension or extension such as 'RV64'. So the word 'extension' is not accurate now, 'feature' seems better.

Differential Revision: https://reviews.llvm.org/D118015