If the linker finds the same name given a BSS and a non-BSS
symbol, the assumption is that the non-BSS symbol is the
true one, and the BSS symbol is just the best Go can do toward
an "extern" declaration. This has always been the case,
as long as the object files were read in the right order.

The old code worked when the BSS symbol is found before
the non-BSS symbol. This CL adds equivalent logic for when
the non-BSS symbol is found before the BSS symbol.
This comes up when Go must refer to symbols defined in
host object files.

LGTM=r
R=r
CC=austin, golang-codereviews, iant, khr
https://golang.org/cl/171480043