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powerpc64: add configure option to select ELFv2 ABI

In preparation for compiler+ABI change in PowerPC64, this change adds a
configure option to select ELFv2 as default powerpc64 ABI when compiling
llvm-devel.

The new option automatically set when ELFv2 environment is detected
(when PPC_ABI=ELFv2)

Submitted by: Alfredo Dal'Ava Júnior
Differential Revision: https://reviews.freebsd.org/D22100