[RISCV] Use const reference when looping over Exts in RISCVISAInfo.

Exts is a map of keyed by std::string with a extension info as
a value. Making copies of this wouldn't be cheap.

We had a mix of references and copies. This makes everything
consistently use a const reference to make it clear we aren't
modifying it.

Differential Revision: https://reviews.llvm.org/D118326