clang/CodeGen/RISCV: test lowering of math builtins (#71399)

Ever since 98c90a13 (ISel: introduce vector ISD::LRINT, ISD::LLRINT;
custom RISCV lowering) landed, there have been several discussions on
how the lrint and llrint libcalls would lower to LLVM IR via clang on
RV32 and RV64, in an effort to enable vectorization of lrint and llrint
via SLPVectorizer and LoopVectorize. This patch adds a new
math-builtins.c test to the RISC-V target to test the lowering of all
math libcalls, including lrint and llrint.