[testsuite] introduce strndup effective target

A number of tests that call strndup fail on vxworks, where there's no
strndup.  Some of them already had workarounds to skip the strndup
parts of the tests on platforms that don't offer it.  I've changed
them to rely on a strndup effective target instead, and extended the
logic to other tests that were otherwise skipped entirely.


for  gcc/ChangeLog

	* doc/sourcebuild.texi (strndup): Add effective target.

for  gcc/testsuite/ChangeLog

	* lib/target-supports.exp (check_effective_target_strndup): New.
	* gcc.dg/builtin-dynamic-object-size-0.c: Skip strndup tests
	when the function is not available.
	* gcc.dg/builtin-dynamic-object-size-1.c: Likewise.
	* gcc.dg/builtin-dynamic-object-size-2.c: Likewise.
	* gcc.dg/builtin-dynamic-object-size-3.c: Likewise.
	* gcc.dg/builtin-dynamic-object-size-4.c: Likewise.
	* gcc.dg/builtin-object-size-1.c: Likewise.
	* gcc.dg/builtin-object-size-2.c: Likewise.
	* gcc.dg/builtin-object-size-3.c: Likewise.
	* gcc.dg/builtin-object-size-4.c: Likewise.