Recover elf32-bigmips and elf32-powerpc support in LLD

This fixes a 7.0 -> 8.0 regression when parsing
OUTPUT_FORMAT("elf32-powerpc"); or elf32-bigmips directive in ldscripts
as well as an unknown emulation error when lld is invoked by clang due
to missed elf32ppclinux case.

Patch by vit9696

Differential Revision: https://reviews.llvm.org/D58005