runtime: fix G passed to schedEnabled and cleanup

exitsyscall0 contains two G variables: _g_ and gp. _g_ is the active G,
g0, while gp is the G to run (which just exited from a syscall).

It is passing _g_ to schedEnabled, which is incorrect; we are about to
execute gp, so that is what we should be checking the schedulability of.

While this is incorrect and should be fixed, I don't think it has ever
caused a problem in practice:

 * g0 does not have g.startpc set, so schedEnabled simplifies to
   just !sched.disable.user.
 * This is correct provided gp is never a system goroutine.
 * As far as I know, system goroutines never use entersyscall /
   exitsyscall.

As far I can tell, this was a simple copy/paste error from exitsyscall,
where variable _g_ is the G to run.

While we are here, eliminate _g_ entirely, as the one other use is
identical to using gp.

Change-Id: I5df98a34569238b89ab13ff7012cd756fefb10dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/291329


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>