[MSan] Make unaligned load/store functions compatible for both endians

[mohit.bhakkad]

Summary

Each member of __msan_retval_tls is of type u64, so assignment of smaller type will be different in each endian.

[eugenis]

This is not right, you can not read uu64 from uu16* in general.
The right way is to mirror what the instrumentation does, i.e.

*(uu16 *)&__msan_retval_tls = *(uu16 *)MEM_TO_SHADOW((uptr)p);

[mohit.bhakkad]

Changed to suggested way, and adding store functions changes.

[eugenis]

LGTM

lib/msan/msan.cc

int __msan_origin_is_descendant_or_same(u32 this_id, u32 prev_id) {
   return o.raw_id() == prev_id;
 }
 
 u32 __msan_get_umr_origin() {
   return __msan_origin_tls;
 }
 
 u16 __sanitizer_unaligned_load16(const uu16 *p) {
-  __msan_retval_tls[0] = *(uu16 *)MEM_TO_SHADOW((uptr)p);
+  __msan_retval_tls[0] = *(uu64 *)MEM_TO_SHADOW((uptr)p);
   if (__msan_get_track_origins())
     __msan_retval_origin_tls = GetOriginIfPoisoned((uptr)p, sizeof(*p));
   return *p;
 }
 u32 __sanitizer_unaligned_load32(const uu32 *p) {
-  __msan_retval_tls[0] = *(uu32 *)MEM_TO_SHADOW((uptr)p);
+  __msan_retval_tls[0] = *(uu64 *)MEM_TO_SHADOW((uptr)p);
   if (__msan_get_track_origins())
     __msan_retval_origin_tls = GetOriginIfPoisoned((uptr)p, sizeof(*p));
   return *p;
 }
 u64 __sanitizer_unaligned_load64(const uu64 *p) {
   __msan_retval_tls[0] = *(uu64 *)MEM_TO_SHADOW((uptr)p);
   if (__msan_get_track_origins())
     __msan_retval_origin_tls = GetOriginIfPoisoned((uptr)p, sizeof(*p));