cmd/ld, pkg/runtime: support generate .o instead of executable

[minux]

discussion: http://golang.org/cl/5822049/

If we support this feature, we don't need to support handing of any relocations, linking
gcc compiled object files.

Pros: able to link in C++ in cgo; remove a lot of code in our linker.
Cons: dependent on gcc for linking cgo programs, even if I could make all gcc compiled
object files available as .syso files.

[rsc]

Comment 1:

It would not be a requirement to use gcc.  We would keep the old code
for linking ordinary C programs.

[alberts]

Comment 2:

Yeah! +inf. this will fix issue #1476.

[alberts]

Comment 3:

Should also fix issue #1781 and issue #4006.

[rsc]

Comment 5:

Oops, created a second one of these.

Status changed to Duplicate.

Merged into issue #4069.