liblink: remove some gotos + other fixes

liblink: remove some gotos + other fixes

Rewrite gotos that violate Go's stricter rules.
Use uchar* instead of char* in a few places that aren't strings.
Remove dead opcross code from asm5.c.
Declare regstr (in both list6 and list8) static.

LGTM=minux, dave
R=minux, dave
CC=golang-codereviews
https://golang.org/cl/113230043