[Darwin, testsuite] Fix Wnonnull on Darwin.

[Darwin, testsuite] Fix Wnonnull on Darwin.

Darwin does not mark entries in string.h with nonnull attributes
so the test fails.  Since the purpose of the test is to check that
the warnings are issued for an inlined function, not that the target
headers are marked up, we can provide marked up headers for Darwin.

gcc/testsuite/ChangeLog:

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: r277202