Compile vismain with -fPIE and link with -pie

Protocted symbol in shared library can only be accessed from PIE
or shared library.  Linker in binutils 2.26 enforces it.  We must
compile vismain with -fPIE and link it with -pie.

	[BZ #17711]
	* elf/Makefile (tests): Add vismain only if PIE is enabled.
	(tests-pie): Add vismain.
	(CFLAGS-vismain.c): New.
	* elf/vismain.c: Add comments for PIE requirement.