While reimplementing config.sub for use in a situation where no bash interpreter was available, I discovered several oddities about config.sub's behavior.
One such oddity was the fact that an explicitly-provided vendor will be clobbered by the inferred vendor for three cpu types: microblaze, s390, and mmix. This commit adds test cases for this clobbering behavior, so that unintentional changes to it will be noticed.
