util: Assume little endian in the absence of platform-specific handling

(cherry picked from commit 6a353479)

Squashed with:

util: Use preprocessor correctly

Fixes: 6a353479 ("util: Assume little endian in the absence of
                      platform-specific handling")
(cherry picked from commit b8cbad62)

Squashed with:

util: Just give up and define PIPE_ARCH_LITTLE_ENDIAN on MSVC

MSVC doesn't support #warning?! Getting really tired of this.

(cherry picked from commit 67676125)

Squashed with:

util: Also include endian.h on cygwin

If u_endian.h can't determine the endianess, the default behaviour in sha1.c
is to build for big-endian

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 2c62ccb1)