cmd/link: in stack bound check, don't check a call repetitively

In stack bound check pass, check a call once, not over and over
again. Fix an accidental quadratic behavior...

In particular, switching to the new linker caused MIPS builders
noticeably slower. This CL fixes it.

Change-Id: Idd00c79e80af6278652c92a1d9d7bb2d194e9490
Reviewed-on: https://go-review.googlesource.com/c/go/+/227078


Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Austin Clements <austin@google.com>