[lld][LinkingContext][ELF] Allow different output file types.

This adds an option --output-filetype that can be set to either
YAML/Native(case insensitive). The linker would create the outputs
associated with the type specified by the user.

Changes all the tests to use the new option.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@191183 91177308-0d34-0410-b5e6-96231b3b80d8