syscall: make func Syscall use pointer maps from Go prototypes

syscall: make func Syscall use pointer maps from Go prototypes

Before, Syscall and friends were having their arguments
treated conservatively. Now they will use the Go prototype,
which will mean the arguments are not considered pointers
at all.

This is safe because of CL 139360044.

The fact that all these non-Solaris systems were using
conservative scanning of the Syscall arguments is why
the failure that prompted CL 139360044 was only
observed on Solaris, which does something completely different.
If we'd done this earlier, we'd have seen the Solaris
failure in more places.

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/144730043