compiler: import inlinable functions from package data

Start reading the export data generated by the last change in this
series.  At this point we will inline direct calls to empty functions
and methods defined in different packages.

Requires this patch to the GCC interface:

>    Index: go-gcc.cc
>    ===================================================================
>    --- go-gcc.cc	(revision 265710)
>    +++ go-gcc.cc	(working copy)
>    @@ -3086,6 +3086,11 @@ Gcc_backend::function(Btype* fntype, con
>	 TREE_THIS_VOLATILE(decl) = 1;
>       if ((flags & function_in_unique_section) != 0)
>	 resolve_unique_section(decl, 0, 1);
>    +  if ((flags & function_only_inline) != 0)
>    +    {
>    +      DECL_EXTERNAL(decl) = 1;
>    +      DECL_DECLARED_INLINE_P(decl) = 1;
>    +    }
>
>       go_preserve_from_gc(decl);
>       return new Bfunction(decl);

Change-Id: I431aae6090ca7bbe381e28d3434e37038e7eddcd
Reviewed-on: https://go-review.googlesource.com/c/150062
Reviewed-by: Than McIntosh <thanm@google.com>
10 files changed