RFR (tedious) 8216022: Use #pragma once

[Florian Weimer]

* Andrew Haley:

> But seriously, how often do you actually have to create a new header
> file, or even edit the part of a header file that touched the include
> guards? The reason this bug hasn't got much love at GCC is that over
> there nobody (including, until today, me) could ever see the point of
> include guards. It seems to me that they are a rather fragile way of
> fixing a nonexistent problem.

The guards occasionally cause bugs because they are not globally unique
or not spelled correctly in both places in the header file.  For
Hotspot, that's probably not a big problem because it's a single code
base, and issues can be fixed easily enough if they arise.

Thanks,
Florian