cmd/link: clean up some relocation handling

cmd/link: clean up some relocation handling

We don't use R_PCREL for calls to dynamic symbols (we use R_CALL
instead). Don't handle R_PCREL as a call.

We don't use R_CALL on ARM64 (we use R_CALLARM64 instead).

Remove those cases, which we don't expect to see.

Change-Id: Idd99022a8eeb65750ffc2936ffdccf8bb0405e30
Reviewed-on: https://go-review.googlesource.com/c/go/+/501859
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>