package/gcc: enable secureplt for powerpc64

package/gcc: enable secureplt for powerpc64

GCC support enabling secureplt for powerpc64.

From [1]
"PowerPC has two PLT models: BSS-PLT and Secure-PLT. BSS-PLT uses
runtime code generation to generate the PLT stubs. Secure-PLT was
introduced with GCC 4.1 and Binutils 2.17 (base has GCC 4.2.1 and
Binutils 2.17), and is a more secure PLT format, using a read-only
linkage table, with the dynamic linker populating a non-executable
index table."

This option is always enabled by glibc testing script
called build-many-glibcs.py [1]. This script exist since
glibc 2.25.

Runtime tested with qemu_ppc64_e5500_defconfig.

[1] https://reviews.freebsd.org/D20598
[2] https://sourceware.org/git/?p=glibc.git;a=blob;f=scripts/build-many-glibcs.py;h=9c08ab7b326e6385abb835eb32dd143952a71942;hb=9826b03b747b841f5fc6de2054bf1ef3f5c4bdf3#l345

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Matt Weber <matthew.weber@collins.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>