uclibc: powerpc: fix PIE/PIC builds with secureplt enabled by default

uclibc: powerpc: fix PIE/PIC builds with secureplt enabled by default

Apply the fix provided by Yann Sionneau when secureplt is enabled
by default by gcc compiler along with PIE/PIC options.

"For the secure PLT to work in PIC, the r30 register needs to point to the GOT"

Fixes:
[qemu_ppc_e500mc_defconfig] https://gitlab.com/buildroot.org/buildroot/-/jobs/1255661606
[qemu_ppc_g3beige_defconfig] https://gitlab.com/buildroot.org/buildroot/-/jobs/1255661607
[qemu_ppc_mac99_defconfig] https://gitlab.com/buildroot.org/buildroot/-/jobs/1255661609

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Yann Sionneau <yann@sionneau.net>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>