Revert "[RISCV] Try using toupper instead of std::toupper to make the build bots happy."
This reverts commit 5311abc7a24e4170e5e6d06f1022da87f1413dd7.
jrtc27 included the proper header in 1bd4986e7cdc124fecbf4b4527039a9a845e61f5
while I was trying to figure out what llvm/clang usually used.