MFC r232491 by tijl:

  Copy amd64 float.h to x86 and merge with i386 float.h. Replace
  amd64/i386/pc98 float.h with stubs.

MFC (part of) r235939 by obrien:

  Consitently use "__LP64__".
  [there are 33 __LP64__'s in the kernel (minus cddl/ and contrib/),
  and 11 _LP64's]

Sponsored by:	The FreeBSD Foundation