[testsuite] Make the Wnonnull test independent of system headers.

[testsuite] Make the Wnonnull test independent of system headers.

This backports two patches that fix failure of Wnonnull test on some targets.

277280 To avoid the result of this test depending on the implementation of
the system 'string.h', provide prototypes for the two functions used
in the test.  This generalises the fix from 277202.

277202 was the initial fix for Wnonnull on Darwin.

gcc/testsuite/

2019-10-30  Iain Sandoe  <iain@sandoe.co.uk>

	Backport from mainline.
	2019-10-22  Iain Sandoe  <iain@sandoe.co.uk>

	* gcc.dg/Wnonnull.c: Provide prototypes for strlen and memcpy.
	Use __SIZE_TYPE__ instead of size_t.

	Backport from mainline.
	2019-10-19  Iain Sandoe  <iain@sandoe.co.uk>

	* gcc.dg/Wnonnull.c: Add attributed function declarations for
	memcpy and strlen for Darwin.

From-SVN: r277647