md5/sha512: fix strict aliasing warnings

md5/sha512: fix strict aliasing warnings

If the target can tolerate these issues, then gcc is smart enough
to generate the same code (x86_64 produces the same code).  If the
target can't, then it needs the memcpy anyways.

libbb/hash_md5_sha.c: In function 'common64_end':
libbb/hash_md5_sha.c:87:4: warning:
  dereferencing type-punned pointer will break strict-aliasing rules
    *(uint64_t *) (&ctx->wbuffer[64 - 8]) = t;
libbb/hash_md5_sha.c: In function 'sha512_end':
libbb/hash_md5_sha.c:886:4: warning:
  dereferencing type-punned pointer will break strict-aliasing rules
    *(uint64_t *) (&ctx->wbuffer[128 - 8]) = t;
libbb/hash_md5_sha.c:889:4: warning:
  dereferencing type-punned pointer will break strict-aliasing rules
    *(uint64_t *) (&ctx->wbuffer[128 - 16]) = t;

Signed-off-by: Mike Frysinger <vapier@gentoo.org>