package/libnss: fix build failure on PowerPC, PowerPC64

package/libnss: fix build failure on PowerPC, PowerPC64

libnss expects OS_TEST to be set to ppc, ppc64, or ppc64le instead of
the usual values defined by Buildroot, respectively powerpc, powerpc64,
or powerpc64le.

This fixes the inconsistencies where functions would be prototyped and
called, but no implementation would be provided, causing link issues.

Define all the variants (of which the existing aarch64be) in Kconfig.
The latest default uses the usual architecture names as defined by
Buildroot, BR2_ARCH.

Additionally, libnss makes use of Altivec intrinsics which are only
available starting with gcc 8, not gcc 5, so we patch it to fix that
condition.

Fixes:
    http://autobuild.buildroot.org/results/579/57928e6cf69d584b430a1d9a99156c57f29b692f/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
[yann.morin.1998@free.fr:
  - move the arch setting to kconfig (like recently done in e3159cad71)
  - add the build failure
  - add pointer to upstream bug report and patch sybmission
  - reformat and reword commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>