The upgrade from 2.12 yielded autopkgtest regressions for cct and
kleborate on s390x, which is notably the only big-endian architecture
with autopkgtest coverage.  In both cases, makeblastdb wound up
determining that it had somehow produced a broken database and
proceeding to bail.

I cannot readily see how other big-endian architectures fare at the
moment, so I'm inclined to add a build dependency on
architecture-is-little-endian for now and leave this bug open as an
invitation to prepare a proper fix.  With this B-D in place, I will
also want to request existing big-endian builds' removal so that 2.16
can migrate to testing.

NB to teammates: I'll want to fold other tuneups into this upload,
namely trying -fsection-anchors on alpha in hopes of keeping GOT sizes
within limits and and dropping to -O on sh4 because disabling
parallelism was insufficient to avoid virtual memory exhaustion.