mcheck: Align struct hdr to MALLOC_ALIGNMENT bytes [BZ #28068]

1. Align struct hdr to MALLOC_ALIGNMENT bytes so that malloc hooks in
libmcheck align memory to MALLOC_ALIGNMENT bytes.
2. Remove tst-mallocalign1 from tests-exclude-mcheck for i386 and x32.
3. Add tst-pvalloc-fortify and tst-reallocarray to tests-exclude-mcheck
since they use malloc_usable_size (see BZ #22057).

This fixed BZ #28068.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

malloc/mcheck.c

@@ -20,6 +20,7 @@
 #ifndef _MALLOC_INTERNAL
 # define _MALLOC_INTERNAL
 # include <malloc.h>
+# include <malloc-size.h>
 # include <mcheck.h>
 # include <stdint.h>
 # include <stdio.h>
@@ -53,7 +54,7 @@ struct hdr
   struct hdr *next;
   void *block;                  /* Real block allocated, for memalign.  */
   unsigned long int magic2;     /* Extra, keeps us doubleword aligned.  */
-};
+} __attribute__ ((aligned (MALLOC_ALIGNMENT)));
 
 /* This is the beginning of the list of all memory blocks allocated.
    It is only constructed if the pedantic testing is requested.  */