cmd/ld: fix PC deltas in DWARF line number table

The putpclcdelta function set the DWARF line number PC to
s->value + pcline->pc, which is correct, but the code then set
the local variable pc to epc, which can be a different value.
This caused the next delta in the DWARF table to be wrong.

Fixes #8098.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/104950045

src/cmd/ld/dwarf.c

@@ -1624,13 +1624,13 @@ writelines(void)
 			}
 			putpclcdelta(s->value + pcline.pc - pc, pcline.value - line);
 
-			pc = epc;
+			pc = s->value + pcline.pc;
+			line = pcline.value;
 			if(pcfile.nextpc < pcline.nextpc)
 				epc = pcfile.nextpc;
 			else
 				epc = pcline.nextpc;
 			epc += s->value;
-			line = pcline.value;
 		}
 
 		da = 0;