[PATCH] testsuite: add coverage for vendor-clobbering

[Adam Joseph]

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.