Suggested by iant@, this change:
  - looks for a symbol _rt0_<GOARCH>_<GOOS>_lib,
  - if the symbol is present, adds a new entry into the .init_array ELF
    section that points to the symbol.

The end-effect is that the symbol _rt0_<GOARCH>_<GOOS>_lib will be
invoked as soon as the (ELF) shared library is loaded, which will in turn
initialize the runtime. (To be implemented.)

Change-Id: I99911a180215a6df18f8a18483d12b9b497b48f4
Reviewed-on: https://go-review.googlesource.com/7692
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>