Add supporting code for runtime initialization, including both
32- and 64-bit x86 architectures.

Add .ctors section on Windows to PE .o files, and INITENTRY to .ctors
section to plug in to the GCC C/C++ startup initialization mechanism.
This allows the Go runtime to initialize itself. Add .text section
symbol for .ctor relocations. Note: This is unlikely to be useful for
MSVC-based toolchains.

Fixes #13494

Change-Id: I4286a96f70e5f5228acae88eef46e2bed95813f3
Reviewed-on: https://go-review.googlesource.com/18057Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>