From Peter Chubb

OK, make it X86 only.  64-bit architectures now get it for free;
if/when PPC32 (the only other 32-bit architecture I can seriously imagine
needing it, except possibly s390) fixes do_div() and needs large block
support, then enable it back there again.

There isn't as far as I know a flag I can test in a config.in file
that distinguishes 64 from 32-bit architectures.  The intention was to
enable the choice of CONFIG_LBD where it made sense;  it doesn't make
sense for 64-bit architectures; and rather than enumerate all the
32-bit architectures, I just chose two that I knew were used in
servers currently.