Fix build without CLANG enabled. [0]

Fix LLD build where PREFIX != LOCALBASE. [1]

powerpc64: add configure option to select ELFv2 ABI [2]

In preparation for compiler+ABI change in PowerPC64, this change adds a
configure option to select ELFv2 as default powerpc64 ABI when compiling
llvm90.

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

PR: 241400 [1], 241410 [0]
Submitted by: Alfredo Dal'Ava Júnior [2]
Differential Revision: https://reviews.freebsd.org/D22100 [2]