cmd/gc: turn Go prototypes into ptr liveness maps for assembly functions

The goal here is to allow assembly functions to appear in the middle
of a Go stack (having called other code) and still record enough information
about their pointers so that stack copying and garbage collection can handle
them precisely. Today, these frames are handled only conservatively.

If you write

        func myfunc(x *float64) (y *int)

(with no body, an 'extern' declaration), then the Go compiler now emits
a liveness bitmap for use from the assembly definition of myfunc.
The bitmap symbol is myfunc.args_stackmap and it contains two bitmaps.
The first bitmap, in effect at function entry, marks all inputs as live.
The second bitmap, not in effect at function entry, marks the outputs
live as well.

In funcdata.h, define new assembly macros:

GO_ARGS opts in to using the Go compiler-generated liveness bitmap
for the current function.

GO_RESULTS_INITIALIZED indicates that the results have been initialized
and need to be kept live for the remainder of the function; it causes a
switch to the second generated bitmap for the assembly code that follows.

NO_LOCAL_POINTERS indicates that there are no pointers in the
local variables being stored in the function's stack frame.

LGTM=khr
R=khr
CC=golang-codereviews
https://codereview.appspot.com/137520043
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://codereview.appspot.com/144730043