runtime: wrap procyield assembly and check for 0

runtime: wrap procyield assembly and check for 0

procyield will currently loop infinitely if passed 0 on several
platforms. This change sidesteps this bug by renaming procyield to
procyieldAsm, and adding a wrapper named procyield that checks for
cycles == 0. The benefit of this structure is that procyield called
with a constant cycle count of 0 will be inlined and constant folded
away, the expected behavior of a procyield of 0 cycles.

A follow-up change will fix the assembly to not have this footgun
anymore.

Change-Id: I7068abfeb961bc0fa475e216836f7c0e46b38373
Reviewed-on: https://go-review.googlesource.com/c/go/+/712663
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>