[dev.link] cmd/link: refer to .got instead of .got.plt on S390X

[dev.link] cmd/link: refer to .got instead of .got.plt on S390X

S390X uses .got instead of .got.plt. It is changed accidentally
in CL 222977. This CL fixes it.

Also, on S390X, we need to set the relocation "variant" of
R_PCREL relocation. In the old code AddPCRelPlus has the magic.
Here we use the equivalent R_PCRELDBL, as the loader doesn't
have variant.

Fix S390X build.

Change-Id: I388e16f02a0568d70287aa9a132fd42b442e3905
Reviewed-on: https://go-review.googlesource.com/c/go/+/223143
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>