[RISCV] Don't use std::vector<std::string> for split extensions in...

[RISCV] Don't use std::vector<std::string> for split extensions in RISCVISAInfo::parseArchString. NFC (#91538)

We can use a SmallVector<StringRef>.

Adjust the code so we check for empty strings in the loop instead of
making a copy of the vector returned from StringRef::split.

This overlaps with #91532 which also removed the std::vector, but
that PR may be more controversial.