cmd/ld: Put the textflag constants in a separate file.
We can then include this file in assembly to replace
cryptic constants like "7" with meaningful constants
like "(NOPROF|DUPOK|NOSPLIT)".

Converting just pkg/runtime/asm*.s for now.  Dropping NOPROF
and DUPOK from lots of places where they aren't needed.
More .s files to come in a subsequent changelist.

A nonzero number in the textflag field now means
"has not been converted yet".

R=golang-dev, daniel.morsing, rsc, khr
CC=golang-dev
https://codereview.appspot.com/12568043