[Ada] Adjust the default MALLOC_OBSERVABLE_ALIGNMENT to 2 * POINTER_SIZE

[Ada] Adjust the default MALLOC_OBSERVABLE_ALIGNMENT to 2 * POINTER_SIZE

Instead of 2 * LONG_TYPE_SIZE. POINTER_SIZE is believed to be the
correct base on more configurations than LONG_TYPE_SIZE and this
adjustment prevents the need for local patches to compensate on
configurations where the latter is inappropriate, for example
x86_64-mingw.

2018-12-03  Olivier Hainque  <hainque@adacore.com>

gcc/ada/

	* gcc-interface/targtyps.c (MALLOC_OBSERVABLE_ALIGNMENT): Set to
	2 * POINTER_SIZE.

From-SVN: r266747