abilist-pattern configurability

This patch creates implicit rules to match the abifiles if
abilist-pattern is defined in the architecture Makefile. This allows
machine specific Makefiles to define different abifiles names
(for instance *-le.abilist for powerpc64le).
PowerPC: Change powerpc64le start ABI to 2.17.
PowerPC: powerpc64le abilist for 2.17

This patch is the abifiles for powerpc64le based on GLIBC 2.17.