[RISCV] Make .option arch parser less mind-bending

Currently the early-return flow in the infinite loop makes it hard to
find the non-error termination points amongst the sea of errors. Rewrite
it with a more conventional control flow that has a clear loop guard (in
place of one of the early returns) and a break (in place of the other),
and with greater code reuse.

This has a small effect on the errors given for malformed input, as seen
in the affected test, and is probably more helpful as a result. Note
that we also bail early now if parseComma fails, as is standard.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D152192