cmd/ld: DWARF opcode base to 10

DWARF 2 has 9 standard opcodes, so dwarfdump expects us to use an
opcode base of at least 10. Previously we used 5.

Discussion:
https://groups.google.com/forum/#!topic/golang-dev/d-BqpPgalzc

LGTM=josharian, rsc
R=golang-codereviews, gobot, rsc, josharian, iant, bradfitz
CC=golang-codereviews
https://golang.org/cl/69320043

src/cmd/ld/dwarf.c

@@ -1412,7 +1412,7 @@ finddebugruntimepath(LSym *s)
 enum {
 	LINE_BASE = -1,
 	LINE_RANGE = 4,
-	OPCODE_BASE = 5
+	OPCODE_BASE = 10
 };
 
 static void
@@ -1541,11 +1541,16 @@ writelines(void)
 	cput(1);   // default_is_stmt
 	cput(LINE_BASE);     // line_base
 	cput(LINE_RANGE);    // line_range
-	cput(OPCODE_BASE);   // opcode_base (we only use 1..4)
+	cput(OPCODE_BASE);   // opcode_base
 	cput(0);   // standard_opcode_lengths[1]
 	cput(1);   // standard_opcode_lengths[2]
 	cput(1);   // standard_opcode_lengths[3]
 	cput(1);   // standard_opcode_lengths[4]
+	cput(1);   // standard_opcode_lengths[5]
+	cput(0);   // standard_opcode_lengths[6]
+	cput(0);   // standard_opcode_lengths[7]
+	cput(0);   // standard_opcode_lengths[8]
+	cput(1);   // standard_opcode_lengths[9]
 	cput(0);   // include_directories  (empty)
 
 	files = emallocz(ctxt->nhistfile*sizeof files[0]);