RFR: 8165440: Add zero support for x86_64-linux-gnux32 target

[John Paul Adrian Glaubitz]

Hi!

This rather small change adds build support for the x86_64-linux-gnux32
target. x32 is a variant of the x86_64 ABI with 64-bit registers and amd64
instruction set but 32-bit pointers [1].

One of the Linux distributions supporting x32 is Debian, see [2]. The
port is not official, but the large majority of packages build fine
and the toolchain is in good shape. Another distribution supporting
x32 is Gentoo.

Please review the change in [3]. I have changed the original patch
a bit that Debian uses downstream to make the changes more consistent
and cleaner.

Thanks,
Adrian

> [1] https://en.wikipedia.org/wiki/X32_ABI
> [2] http://debian-x32.org/
> [2] http://cr.openjdk.java.net/~glaubitz/8165440/webrev.00/

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer - glaubitz at debian.org
`. `'   Freie Universitaet Berlin - glaubitz at physik.fu-berlin.de
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