cmd/5l, cmd/6l, cmd/8l: accept CALL reg, reg

The new src argument is ignored during linking
(that is, CALL r1, r2 is identical to CALL r2 for linking),
but it serves as a hint to the 5g/6g/8g optimizer
that the src register is live on entry to the called
function and must be preserved.

It is possible to avoid exposing this fact to the rest of
the toolchain, keeping it entirely within 5g/6g/8g,
but I think it will help to be able to look in object files
and assembly listings and linker -a / -W output to
see CALL instructions are "Go func value" calls and
which are "C function pointer" calls.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7364045
cmd/gc: avoid runtime code generation for closures

Change ARM context register to R7, to get out of the way
of the register allocator during the compilation of the
prologue statements (it wants to use R0 as a temporary).

Step 2 of http://golang.org/s/go11func.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7369048
exp/ssa: support variadic synthetic methods.

We wrap the final '...' argument's type in types.Slice.
Added tests.

Also:
- Function.writeSignature: suppress slice '[]' when printing
  variadic arg '...'.
- Eliminate Package.ImportPath field; redundant
  w.r.t. Package.Types.Path.
- Use "TODO: (opt|fix)" notation more widely.
- Eliminate many redundant/stale TODOs.

R=gri
CC=golang-dev
https://codereview.appspot.com/7378057