Unify the 386 entry point code as much as possible.

The main function could not be unified because on Windows 386 it is
called _main. Putting main in asm_386.s caused multiple definition
errors when using the external linker.

Add the _lib entry point to various operating systems.  A future CL
will enable c-archive/c-shared mode for those targets.

Fix _rt0_386_windows_lib_go--it was passing arguments as though it
were amd64.

Change-Id: Ic73f1c95cdbcbea87f633f4a29bbc218a5db4f58
Reviewed-on: https://go-review.googlesource.com/70530
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>