liblink, runtime: preliminary support for plan9/amd64

A TLS slot is reserved by _rt0_.*_plan9 as an automatic and
its address (which is static on Plan 9) is saved in the
global _privates symbol. The startup linkage now is exactly
like that from Plan 9 libc, and the way we access g is
exactly as if we'd have used privalloc(2).

Aside from making the code more standard, this change
drastically simplifies it, both for 386 and for amd64, and
makes the Plan 9 code in liblink common for both 386 and
amd64.

The amd64 runtime code was cleared of nxm assumptions, and
now runs on the standard Plan 9 kernel.

Note handling fixes will follow in a separate CL.

LGTM=rsc
R=golang-codereviews, rsc, bradfitz, dave
CC=0intro, ality, golang-codereviews, jas, minux.ma, mischief
https://golang.org/cl/101510049

include/link.h

@@ -391,7 +391,7 @@ struct	Link
 	LSym*	sym_modu;
 	LSym*	symmorestack[20];
 	LSym*	tlsg;
-	LSym*	plan9tos;
+	LSym*	plan9privates;
 	Prog*	curp;
 	Prog*	printp;
 	Prog*	blitrl;