i386: Increase MALLOC_ALIGNMENT to 16 [BZ #21120]

i386: Increase MALLOC_ALIGNMENT to 16 [BZ #21120]

GCC 7 changed the definition of max_align_t on i386:

https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=9b5c49ef97e63cc63f1ffa13baf771368105ebe2

As a result, glibc malloc no longer returns memory blocks which are as
aligned as max_align_t requires.

This causes malloc/tst-malloc-thread-fail to fail with an error like this
one:

error: allocation function 0, size 144 not aligned to 16

This patch moves the MALLOC_ALIGNMENT definition to <malloc-alignment.h>
and increases the malloc alignment to 16 for i386.

	[BZ #21120]
	* malloc/malloc-internal.h (MALLOC_ALIGNMENT): Moved to ...
	* sysdeps/generic/malloc-alignment.h: Here.  New file.
	* sysdeps/i386/malloc-alignment.h: Likewise.
	* sysdeps/generic/malloc-machine.h: Include <malloc-alignment.h>.