ELF: Ignore argument after --plugin.

Clang passes --plugin /path/to/LLVMgold.so to the linker when -flto is
passed. After r333607 we only ignore --plugin as a joined argument,
which means that the following argument (/path/to/LLVMgold.so) is
interpreted as an input file. This means that either every LTO'd
program ends up being linked with the gold plugin or we error out
if the plugin does not exist. The fix is to use Eq to ignore both
--plugin=foo and --plugin foo as before.

Differential Revision: https://reviews.llvm.org/D47657

llvm-svn: 333793