liblink: remove arch-specific constants from file format

liblink: remove arch-specific constants from file format

The relocation and automatic variable types were using
arch-specific numbers. Introduce portable enumerations
instead.

To the best of my knowledge, these are the only arch-specific
bits left in the new object file format.

Remove now, before Go 1.3, because file formats are forever.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/87670044