Update LTO plugin interface

[Ian Lance Taylor]

"H.J. Lu" <hjl.tools@gmail.com> writes:

> __udivdi3 is just an example.  It can also happen to memcpy, or
> any library calls generated by GCC. I am enclosing a testcase for memcpy.

I believe we can solve that specific problem much more efficiently than
requiring a complete link of all the input files.  We currently solve it
using the -pass-through option which is passed to the linker plugin.
Are there any cases for which using -pass-through=-lc
-pass-through=-lgcc would not be a complete solution?

Ian