8l, runtime: fix Plan 9 386 build.

8l, runtime: fix Plan 9 386 build.
8l was broken by commit 7ac0d2eed9, it caused .data to be page aligned in the file - which is not how Plan 9 expects things to be.
Also .rodata was layed out in a similar fashion.

Not sure when signame was introduced, but added a stub.
Removed the symo assignment in asm.c as it is not currently used.

Fix runtime breakage after commit 629c065d36 which prefixes all external symbols with runtime·.

R=rsc
CC=golang-dev
https://golang.org/cl/2674041