cmd/link/internal/sym: uncomment code for ELF cases in RelocName

cmd/link/internal/sym: uncomment code for ELF cases in RelocName

When this code was introduced, there were no R_MIPS, R_PPC64 and
R_390 and build would fail with this code uncommented.
Now we have those.

Change-Id: I18a54eaa250db12e293f8e4d1f080f1dd2e66a4f
Reviewed-on: https://go-review.googlesource.com/122896
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>