Add the -Xlinker option to bugpoint which allows an option to be passed

Add the -Xlinker option to bugpoint which allows an option to be passed
through to gcc when its being used as a linker. This allows -L and -l
(and any other) options to be added so that non-complete bytecode files
can be processed with bugpoint. The -Xlinker option can be added as many
times as needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28692 91177308-0d34-0410-b5e6-96231b3b80d8