https://golang.org/cl/5822049 introduced the idea of linking together
all the cgo objects with -r, while also linking against -lgcc. This
was to fix http://golang.org/issue/3261: cgo code that requires libgcc
would break when using internal linking.

This approach introduced https://golang.org/issue/9510: multiple
different cgo packages could include the same libgcc object, leading
to a multiple definition error during the final link. That problem was
fixed by https://golang.org/cl/16741, as modified by
https://golang.org/cl/16993, which did the link against libgcc only
during the final link.

After https://golang.org/cl/16741, and, on Windows, the later
https://golang.org/cl/26670, ld -r no longer does anything useful.

So, remove it.

Doing this revealed that running ld -r on Darwin simplifies some
relocs by making them specific to a symbol rather than a section.
Correct the handling of unsigned relocations in internal linking mode
by offsetting by the symbol value. This only really comes up when
using the internal linker with C code that initializes a variable to
the address of a local constant, such as a C string (as in const char
*s = "str";). This change does not affect the normal case of external
linking, where the Add field is ignored. The test case is
misc/cgo/test/issue6612.go in internal linking mode.

The cmd/internal/goobj test can now see an external object with no
symbol table; fix it to not crash in that case.

Change-Id: I15e5b7b5a8f48136bc14bf4e1c4c473d5eb58062
Reviewed-on: https://go-review.googlesource.com/64793
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>