MIPS: Make sure callee-saved registers end up on word boundaries

In addition to handling parameters and local variables,
mono_arch_allocate_vars reserves space for callee-saved registers.

Even though the current implementation takes care of properly aligning
the variables according to their datatype, it doesn't force alignment
to a word boundary before processing the callee-saved area.  This can
result in misaligned accesses if the last locals are booleans or small
integers.

Worse still: the garbage collector does then not see these non-word-
aligned references, and can end up reclaiming the pointed-to objects
if there are no other references to them--causing strange crashes and
type morphing effects under memory pressure.

Fix that by forcibly aligning the callee-saved area.

Commit contributed under the MIT/X11 license.

mono/mini/mini-mips.c

@@ -1328,6 +1328,8 @@ mono_arch_allocate_vars (MonoCompile *cfg)
 	 * args or return vals.  Extra stack space avoids this in a lot of cases.
 	 */
 	offset += EXTRA_STACK_SPACE;
+	offset += SIZEOF_REGISTER - 1;
+	offset &= ~(SIZEOF_REGISTER - 1);
 
 	/* Space for saved registers */
 	cfg->arch.iregs_offset = offset;