[llvm][RISCV] Improve error message for invalid extension letters (#90468)

Previously you got:
clang: error: invalid arch name 'rv64v', first letter should be 'e', 'i'
or 'g'

Which to me, unfamiliar with riscv, reads as if I should have used
"[eig]rv64v". Which is not what clang means.

Include the first bit in the error message to make this clearer:
clang: error: invalid arch name 'rv64v', first letter after 'rv64'
should be 'e', 'i' or 'g'