liblink, sync/atomic: fix arm build

The liblink code to insert the FUNCDATA for a stack map
from the Go prototype was not correct for ARM
(different data structure layout).

Also, sync/atomic was missing some Go prototypes
for ARM-specific functions.

TBR=r
CC=golang-codereviews
https://golang.org/cl/143160045

include/link.h

@@ -471,6 +471,7 @@ struct LinkArch
 	int	D_PARAM;
 	int	D_SCONST;
 	int	D_STATIC;
+	int	D_OREG;
 
 	int	ACALL;
 	int	ADATA;

src/liblink/obj5.c

@@ -1061,6 +1061,7 @@ LinkArch linkarm = {
 	.D_PARAM = D_PARAM,
 	.D_SCONST = D_SCONST,
 	.D_STATIC = D_STATIC,
+	.D_OREG = D_OREG,
 
 	.ACALL = ABL,
 	.ADATA = ADATA,

src/liblink/objfile.c

@@ -268,7 +268,12 @@ writeobj(Link *ctxt, Biobuf *b)
 			p->as = ctxt->arch->AFUNCDATA;
 			p->from.type = ctxt->arch->D_CONST;
 			p->from.offset = FUNCDATA_ArgsPointerMaps;
-			p->to.type = ctxt->arch->D_EXTERN;
+			if(ctxt->arch->thechar == '6' || ctxt->arch->thechar == '8')
+				p->to.type = ctxt->arch->D_EXTERN;
+			else {
+				p->to.type = ctxt->arch->D_OREG;
+				p->to.name = ctxt->arch->D_EXTERN;
+			}
 			p->to.sym = linklookup(ctxt, smprint("%s.args_stackmap", s->name), s->version);
 		}
 	}

src/sync/atomic/64bit_arm.go

@@ -44,3 +44,9 @@ func swapUint64(addr *uint64, new uint64) (old uint64) {
 	}
 	return
 }
+
+// Additional ARM-specific assembly routines.
+// Declaration here to give assembly routines correct stack maps for arguments.
+func armCompareAndSwapUint32(addr *uint32, old, new uint32) (swapped bool)
+func armCompareAndSwapUint64(addr *uint64, old, new uint64) (swapped bool)
+func generalCAS64(addr *uint64, old, new uint64) (swapped bool)