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>

compat/bswap.h

@@ -149,11 +149,12 @@ static inline uint64_t git_bswap64(uint64_t x)
  * and is faster on architectures with memory alignment issues.
  */
 
-#if defined(__i386__) || defined(__x86_64__) || \
+#if !defined(NO_UNALIGNED_LOADS) && ( \
+    defined(__i386__) || defined(__x86_64__) || \
     defined(_M_IX86) || defined(_M_X64) || \
     defined(__ppc__) || defined(__ppc64__) || \
     defined(__powerpc__) || defined(__powerpc64__) || \
-    defined(__s390__) || defined(__s390x__)
+    defined(__s390__) || defined(__s390x__))
 
 #define get_be16(p)	ntohs(*(unsigned short *)(p))
 #define get_be32(p)	ntohl(*(unsigned int *)(p))