Fix new testcase elf/tst-latepthread on s390x.

Fix new testcase elf/tst-latepthread on s390x.

The first dlopen ("tst-latepthreadmod.so", RTLD_LOCAL | RTLD_LAZY) call
in elf/tst-latepthread.c fails on s390x with "error: dlopen failed:
.../build-s390x/elf/tst-latepthreadmod.so:
undefined symbol: this_function_is_not_defined".

In elf/tst-latepthreadmod.c, this_function_is_not_defined is a sibling
call which leads to a R_390_GLOB_DAT relocation in .rela.dyn instead of
a R_390_JMP_SLOT in .rela.plt.

As RTLD_LAZY skips the JMP_SLOT relocations, but not GLOB_DAT ones,
the dlopen call fails. If elf/tst-latepthreadmod.c is build with
-fno-optimize-sibling-calls, a JMP_SLOT relocation is generated for
this_function_is_not_defined and the test passes.

ChangeLog:

	* elf/Makefile (CFLAGS-tst-latepthreadmod.c):
	Add -fno-optimize-sibling-calls.