Add a test for prelink output

This test applies to i386 and x86_64 which set R_386_GLOB_DAT and
R_X86_64_GLOB_DAT to ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA.

	[BZ #19178]
	* sysdeps/x86/Makefile (tests): Add tst-prelink.
	(tst-prelink-ENV): New.
	($(objpfx)tst-prelink-conflict.out): Likewise.
	($(objpfx)tst-prelink-cmp.out): Likewise.
	(tests-special): Add $(objpfx)tst-prelink-cmp.out.
	* sysdeps/x86/tst-prelink.c: New file.
	* sysdeps/x86/tst-prelink.exp: Likewise.