powerpc: Fix ifuncmain6pie failure with GCC 4.9

powerpc: Fix ifuncmain6pie failure with GCC 4.9

This patch fix the elf/ifuncmain6pie failure when building with GCC
4.9+.  For some reason, the compiler removes the branch taken code at
resolve_ifunc (sysdeps/powerpc/powerpc64/dl-machine.h) as dead-code
and thus the testcase fails because the ifunc resolves branches to an
invalid memory location.  It fixes by explicit adding a dependency of
value based on odp variable to avoid compiler optimization.

It fixes BZ#17868.