[Ada] Use standard version of s-memory.adb for mingw32

[Ada] Use standard version of s-memory.adb for mingw32

This patch switches mingw32 targets to use the standard version of s-memory.adb
as Windows now has the capability of limiting the amount of memory used by
process.

2018-07-17  Patrick Bernardi  <bernardi@adacore.com>

gcc/ada/

	* libgnat/s-memory__mingw.adb: Remove.
	* Makefile.rtl: Remove s-memory.adb target pair from the Cygwin/Mingw32
	section.

gcc/testsuite/

	* gnat.dg/memorytest.adb: New testcase.

From-SVN: r262796