cmd/ld: fix off-by-one error in DWARF .debug_line transcription

The liblink refactor changed the DWARF .debug_line flow control. The mapping was off by one pcline entry. The fix here preserves pc until it can be compared to pcline.pc.

Sample dwarfdump .debug_line output for main.main from the program in issue 7351, before liblink (correct):

0x0000003c: 00 Extended: <9> 02 DW_LNE_set_address( 0x0000000000002000 )
0x00000047: 03 DW_LNS_advance_line( 6 )
0x00000049: 01 DW_LNS_copy
            0x0000000000002000      1      7      0 is_stmt

0x0000004a: 8b address += 21,  line += 1
            0x0000000000002021      1      8      0 is_stmt

0x0000004b: 02 DW_LNS_advance_pc( 153 )
0x0000004e: 03 DW_LNS_advance_line( 1 )
0x00000050: 01 DW_LNS_copy
            0x00000000000020ba      1      9      0 is_stmt

After liblink (off by one entry):

0x00001bbf: 00 Extended: <9> 02 DW_LNE_set_address( 0x0000000000002000 )
0x00001bca: 02 DW_LNS_advance_pc( 33 )
0x00001bcc: 03 DW_LNS_advance_line( 6 )
0x00001bce: 01 DW_LNS_copy
            0x0000000000002021      1      7      0 is_stmt

0x00001bcf: 02 DW_LNS_advance_pc( 153 )
0x00001bd2: 03 DW_LNS_advance_line( 1 )
0x00001bd4: 01 DW_LNS_copy
            0x00000000000020ba      1      8      0 is_stmt

0x00001bd5: 02 DW_LNS_advance_pc( 153 )
0x00001bd8: 03 DW_LNS_advance_line( 1 )
0x00001bda: 01 DW_LNS_copy
            0x0000000000002153      1      9      0 is_stmt

After this CL (the line 9 pc offset changed due to intervening compiler changes):

0x00001d07: 00 Extended: <9> 02 DW_LNE_set_address( 0x0000000000002000 )
0x00001d12: 03 DW_LNS_advance_line( 6 )
0x00001d14: 01 DW_LNS_copy
            0x0000000000002000      1      7      0 is_stmt

0x00001d15: 8b address += 21,  line += 1
            0x0000000000002021      1      8      0 is_stmt

0x00001d16: 02 DW_LNS_advance_pc( 189 )
0x00001d19: 03 DW_LNS_advance_line( 1 )
0x00001d1b: 01 DW_LNS_copy
            0x00000000000020de      1      9      0 is_stmt

Fixes #7351.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/66290043

src/cmd/ld/dwarf.c

@@ -1590,29 +1590,30 @@ writelines(void)
 
 		pciterinit(&pcfile, &s->pcln->pcfile);
 		pciterinit(&pcline, &s->pcln->pcline);
+		epc = pc;
 		while(!pcfile.done && !pcline.done) {
-			if(pc - s->value >= pcfile.nextpc) {
+			if(epc - s->value >= pcfile.nextpc) {
 				pciternext(&pcfile);
 				continue;
 			}
-			if(pc - s->value >= pcline.nextpc) {
+			if(epc - s->value >= pcline.nextpc) {
 				pciternext(&pcline);
 				continue;
 			}
 
-			if(pcfile.nextpc < pcline.nextpc)
-				epc = pcfile.nextpc;
-			else
-				epc = pcline.nextpc;
-			epc += s->value;
-
 			if(file != pcfile.value) {
 				cput(DW_LNS_set_file);
 				uleb128put(pcfile.value);
 				file = pcfile.value;
 			}
-			putpclcdelta(epc - pc, pcline.value - line);
+			putpclcdelta(s->value + pcline.pc - pc, pcline.value - line);
+
 			pc = epc;
+			if(pcfile.nextpc < pcline.nextpc)
+				epc = pcfile.nextpc;
+			else
+				epc = pcline.nextpc;
+			epc += s->value;
 			line = pcline.value;
 		}