cmd/internal/obj/x86: add named consts for Prog.Back flags
cmd/internal/obj/x86: add named consts for Prog.Back flags

Passes toolstash-check.

Change-Id: I7232ba43895a54b03cc328bb2e99c05c44eb6e8e
Reviewed-on: https://go-review.googlesource.com/107057
Reviewed-by: Ilya Tocar <112830fdd81fb6e3c10661fec6ab883a8a8a909f@intel.com>
Run-TryBot: Iskander Sharipov <a698043bc2fd5ebc21d5b6bbc66ee53917362fef@intel.com>

golang/go,nawawi/go,josharian/go,nawawi/go,golang/go,CAFxX/go,golang/go,akutz/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,akutz/go,josharian/go,golang/go,josharian/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,josharian/go,akutz/go,josharian/go,cloudflare/go,golang/go,josharian/go,CAFxX/go,josharian/go,akutz/go,akutz/go,nawawi/go,cloudflare/go,josharian/go,akutz/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,akutz/go