MFV r302649: 7016 arc_available_memory is not 32-bit safe

MFV r302649: 7016 arc_available_memory is not 32-bit safe

illumos/illumos-gate@0dd053d7d890618ea1fc697b07de364e69eb4190
https://github.com/illumos/illumos-gate/commit/0dd053d7d890618ea1fc697b07de364e69eb4190

https://www.illumos.org/issues/7016
  upstream DLPX-39446 arc_available_memory is not 32-bit safe
  https://github.com/delphix/delphix-os/commit/
  6b353ea3b8a1610be22e71e657d051743c64190b
  related to this upstream:
  DLPX-38547 delphix engine hang
  https://github.com/delphix/delphix-os/commit/
  3183a567b3e8c62a74a65885ca60c86f3d693783
  DLPX-38547 delphix engine hang (fix static global)
  https://github.com/delphix/delphix-os/commit/
  22ac551d8ef085ad66cc8f65e51ac372b12993b9
  DLPX-38882 system hung waiting on free segment
  https://github.com/delphix/delphix-os/commit/
  cdd6beef7548cd3b12f0fc0328eeb3af540079c2

Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Gordon Ross <gordon.ross@nexenta.com>
Author: Prakash Surya <prakash.surya@delphix.com>