runtime: remove remaining use of MAKECONTEXT_STACK_TOP macro
runtime: remove remaining use of MAKECONTEXT_STACK_TOP macro

The definition and most uses of MAKECONTEXT_STACK_TOP were removed in
https://golang.org/cl/88660043, which removed support for Solaris 8/9.
One use of MAKECONTEXT_STACK_TOP was accidentally left in the source
code.  Remove it now.

Change-Id: I78f845d71ff54bda35a3e66e66b9231be6d76f7a
Reviewed-on: https://go-review.googlesource.com/28911
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend