Newer compiler versions, like GCC 10 and Clang 12, have built-in
functions for bswap32 and bswap64. This comes in handy, for example,
when targeting CLANGARM64 on Windows, which would not be supported
without this logic.
Signed-off-by: Dennis Ameling <dennis@dennisameling.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>