bswap: add NO_UNALIGNED_LOADS define

The byte-swapping code automatically decides, based on the platform, whether it is sensible to cast and do a potentially unaligned ntohl(), or to pick individual bytes out of an array. It can be handy to override this decision, though, when turning on compiler flags that will complain about unaligned loads (such as -fsanitize=undefined). This patch adds a macro check to make this possible. There's no nice Makefile knob here; this is for prodding at Git's internals, and anybody using it can set "-DNO_UNALIGNED_LOADS" in the same place they are setting up "-fsanitize". Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>