runtime: cleanup: use ArgsSizeUnknown to mark all functions

runtime: cleanup: use ArgsSizeUnknown to mark all functions
whose argument size is unknown (C vararg functions, and
assembly code without an explicit specification).

We used to use 0 to mean "unknown" and 1 to mean "zero".
Now we use ArgsSizeUnknown (0x80000000) to mean "unknown".

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/11590043