tuklib_integer: Fix building on OpenBSD/sparc64 that uses GCC 4.2

tuklib_integer: Fix building on OpenBSD/sparc64 that uses GCC 4.2

GCC 4.2 doesn't have __builtin_bswap16() and friends so tuklib_integer.h
tries to use OS-specific byte swap methods instead. On OpenBSD those
macros are swap16/32/64 instead of bswap16/32/64 like on other *BSDs
and Darwin.

An alternative to "#ifdef __OpenBSD__" could be "#ifdef swap16" as it
is a macro. But since OpenBSD seems to be a special case under this
special case of "*BSDs and Darwin", checking for __OpenBSD__ seems
the more conservative choice now.

Thanks to Christian Weisgerber and Brad Smith who both submitted
the same patch a few hours apart.

Co-authored-by: Christian Weisgerber <naddy@mips.inka.de>
Co-authored-by: Brad Smith <brad@comstyle.com>
Closes: https://github.com/tukaani-project/xz/pull/126
(cherry picked from commit 04b23addf3733873667675df2439725f076c2f36)