This single patch is effectively a no-op for now. It enables architectures

to opt in to using GCC's __builtin_bswapXX() intrinsics for byteswapping,
and if we merge this now then the architecture maintainers can enable it
for their arch during the next cycle without dependency issues.

It's worth making it a par-arch opt-in, because although in *theory* the
compiler should never do worse than hand-coded assembler (and of course
it also ought to do a lot better on platforms like Atom and PowerPC which
have load-and-swap or store-and-swap instructions), that isn't always the
case. See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46453 for example.
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