Compile tst-prelink.c without PIE [BZ #21815]

tst-prelink.c checks for conflict with GLOB_DAT relocation against stdio.
On i386, there is no GLOB_DAT relocation against stdio with PIE.  We
should compile tst-prelink.c without PIE.

	[BZ #21815]
	* elf/Makefile (CFLAGS-tst-prelink.c): New.
	(LDFLAGS-tst-prelink): Likewise.