pv-runtime: Don't warn if library aliases are missing from secondary archs

• pv-supported-architectures: 0'th architecture is primary

  On x86, the 0'th architecture is hard-coded to be x86_64. On other platforms, there is only one architecture, which is whatever we are compiling for.

• tests: Assert that the table of supported architectures is consistent

• pv-runtime: Don't warn if library aliases are missing from secondary archs

  Our older runtimes are symmetrical, so the vast majority of libraries (and in particular, the ones that need aliases for an alternate SONAME) are available for both x86_64 and i386.

  However, in some of our newer runtimes like medic, we've started dropping i386 libraries that don't strictly need to exist in order to save some space. This means we can't necessarily assume that libraries that need aliases will exist for every architecture, so we shouldn't emit warnings if they turn out not to exist.

  steamrt/tasks#367

/cc @refi64