[RISCV][llvm-mca] Add mca tests for riscv lmul instruments

Add llvm-mca tests for RISCV LMUL instruments to show that llvm-mca RISCV LMUL
instruments work.

This commit was previously reverted in ad8765a0. This commit adds a lit.local.cfg
to make RISCV llvm-mca tests unsupported on non-RISCV configurations.

Differential Revision: https://reviews.llvm.org/D149496