amd64 buildworld cleanup

["Simon 'corecode' Schubert" <corecode@xxxxxxxxxxxx>]

hey,

i sat down and fixed various issues so that a TARGET_ARCH=amd64 
buildworld run can now be completed.

Fetch the patch from <http://leaf.dragonflybsd.org/~corecode/amd64.diff> 
 and please review and comment.  I'd like to commit this in, say, 3 
days if nobody objects (possibly in smaller chunks).

Most of the patch is amd64 changes/additions, but some also affect i386 
(a short test showed no regressions).

Changes are heavily based on FreeBSD's amd64 port (with the exception of 
libc's math support, which is by NetBSD).  I want to thank the people 
who put so much effort in it and make it really easy for us to pick up a 
new port!

cheers
  simon

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