[utils] Add RISC-V support to update_llc_test_checks.py

This should be a trivial change, and I've started using it for generating all 
tests at https://github.com/lowrisc/riscv-llvm (i.e. it's been tested in 
action quite a lot). Note that the regex does not attempt to match 
.cfi_startproc, as I want to ensure compatibility with functions that have the 
nounwind attribute.

Differential Revision: https://reviews.llvm.org/D39789

llvm-svn: 317693