[JITLink] Add support for passing arguments to jit-linked code.

The --args option can now be used to pass arguments to code linked with
llvm-jitlink. E.g.

$ llvm-jitlink file1.o file2.o --args a b c

is equivalent to:

$ ld -o program file1.o file2.o
$ ./program a b c

llvm-svn: 359115