runtime: clear Defer.panic before removing from G.defer list

Another dangling stack pointer in a cached structure.
Same as SudoG.elem and SudoG.selectdone.

Definitely a fix, and the new test in freedefer makes the
crash reproducible, but probably not a complete fix.
I have seen one dangling pointer in a Defer.panic even
after this fix; I cannot see where it could be coming from.

I think this will fix the solaris build.
I do not think this will fix the occasional failure on the darwin build.

TBR=khr
R=khr
CC=golang-codereviews
https://codereview.appspot.com/155080043
runtime: change Windows M.thread from void* to uintptr

It appears to be an opaque bit pattern more than a pointer.
The Go garbage collector has discovered that for m0
it is set to 0x4c.

Should fix Windows build.

TBR=brainman
CC=golang-codereviews
https://codereview.appspot.com/149640043