[RISCV] Add error checking for extensions missing separating underscores

Currently if two multi-letter extensions are provided in a -march=
string, the verification code checks the version of the first and
consumes the second, resulting in that part of the architecture
string being ignored. This adds a test that when a version number has
been parsed for an extension, there are no subsequent characters.

Differential Revision: https://reviews.llvm.org/D83819