[dev.cc] liblink: don't patch jumps to jumps to symbols

When liblink sees something like

       JMP x
       ...
    x: JMP y

it rewrites the first jump to jump directly to y.  This is
fine if y is a resolved label.  However, it *also* does this
if y is a function symbol, but fails to carry over the
relocation that would later patch in that symbol's value.  As
a result, the original jump becomes either a self-jump (if
relative) or a jump to PC 0 (if absolute).

Fix this by disabling this optimization if the jump being
patched in is a jump to a symbol.

LGTM=minux
R=rsc, minux
CC=golang-codereviews
https://golang.org/cl/185890044

src/liblink/pass.c

@@ -41,7 +41,7 @@ brchain(Link *ctxt, Prog *p)
 	int i;
 
 	for(i=0; i<20; i++) {
-		if(p == nil || p->as != ctxt->arch->AJMP)
+		if(p == nil || p->as != ctxt->arch->AJMP || p->pcond == nil)
 			return p;
 		p = p->pcond;
 	}
@@ -56,7 +56,7 @@ brloop(Link *ctxt, Prog *p)
 
 	c = 0;
 	for(q = p; q != nil; q = q->pcond) {
-		if(q->as != ctxt->arch->AJMP)
+		if(q->as != ctxt->arch->AJMP || q->pcond == nil)
 			break;
 		c++;
 		if(c >= 5000)