compat: add endianness helpers

compat: add endianness helpers

The POSIX standard doesn't currently define a `ntohll`/`htonll`
function pair to perform network-to-host and host-to-network
swaps of 64-bit data. These 64-bit swaps are necessary for the on-disk
storage of EWAH bitmaps if they are not in native byte order.

Many thanks to Ramsay Jones <redacted> and
Torsten Bögershausen <redacted> for cygwin/mingw/msvc
portability fixes.

Signed-off-by: Vicent Marti <redacted>
Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>