Check linker support for INSERT in linker script

Since gold doesn't support INSERT in linker script:

https://sourceware.org/bugzilla/show_bug.cgi?id=21676

tst-split-dynreloc fails to link with gold.  Check if linker supports
INSERT in linker script before using it.

	* config.make.in (have-insert): New.
	* configure.ac (libc_cv_insert): New.  Set to yes if linker
	supports INSERT in linker script.
	(AC_SUBST(libc_cv_insert): New.
	* configure: Regenerated.
	* sysdeps/x86_64/Makefile (tests): Add tst-split-dynreloc only
	if $(have-insert) == yes.