[Gnu] Set the type of binary that lld would generate.

This is needed before any of the search paths are searched for.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195596 91177308-0d34-0410-b5e6-96231b3b80d8
[Gnu] Ignore unknown arguments, and print message.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195597 91177308-0d34-0410-b5e6-96231b3b80d8
[Gnu] Set the defaults in the ELFLinkingContext.

Comment from Rui Ueyema.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195598 91177308-0d34-0410-b5e6-96231b3b80d8