Introduce a new linker command line option "-strictdups", which
enables sanity checking of "ok to duplicate" symbols, especially
DWARF info symbols. Acceptable values are 0 (no checking) 1 (issue
warnings) and 2 (issue a fatal error checks fail).

Currently if we read a DWARF symbol (such as "go.info.PKG.FUNCTION")
from one object file, and then encounter the same symbol later on
while reading another object file, we simply discard the second one
and move on with the link, since the two should in theory be
identical.

If as a result of a compiler bug we wind up with symbols that are not
identical, this tends to (silently) result in incorrect DWARF
generation, which may or may not be discovered depending on who is
consuming the DWARF and what's being done with it.

When this option is turned on, at the point where a duplicate
symbol is detected in the object file reader, we check to make sure
that the length/contents of the symbol are the same as the previously
read symbol, and print a descriptive warning (or error) if not.

For the time being this can be used for one-off testing to find
problems; at some point it would be nice if we can enable it by
default.

Updates #30908.

Change-Id: I64c4e07c326b4572db674ff17c93307e2eec607c
Reviewed-on: https://go-review.googlesource.com/c/go/+/168410
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>