This libgo patch by Cherry Zhang changes the runtime package to call
dropg before changing the  g status to _Grunnable/_Gwaiting.
Currently, we dropg (which clears gp.m) after we CAS the g status to
_Grunnable or _Gwaiting.  Immediately after CASing the g status,
another thread may CAS it to _Gscan status and scan its stack.  With
precise stack scan, it accesses gp.m in order to switch to g and back
(in doscanstackswitch).  This races with dropg.  If doscanstackswitch
reads gp.m, then dropg runs, when we restore the m at the end of the
scan it will set to a stale value.  Worse, if dropg runs after
doscanstackswitch sets the new m, gp will be running with a nil m.

To fix this, we do dropg before CAS g status to _Grunnable or
_Gwaiting.  We can do this safely if we are CASing from _Grunning, as
we own the g when it is in _Grunning.  There is one case where we CAS
from _Gsyscall to _Grunnable.  It is not safe to dropg when it is in
_Gsyscall, as precise stack scan needs to read gp.m in order to signal
the m.  So we need to introduce a transient state, _Gexitingsyscall,
between _Gsyscall and _Grunnable, where the GC should not scan its
stack.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian