dwarf: add extensions for multi-file compression (.dwz)

LGTM=iant
R=golang-codereviews, iant, bradfitz
CC=golang-codereviews, math-nuts
https://golang.org/cl/72820044

src/pkg/debug/dwarf/const.go

@@ -212,6 +212,10 @@ const (
 	formExprloc     format = 0x18
 	formFlagPresent format = 0x19
 	formRefSig8     format = 0x20
+	// Extensions for multi-file compression (.dwz)
+	// http://www.dwarfstd.org/ShowIssue.php?issue=120604.1
+	formGnuRefAlt  format = 0x1f20
+	formGnuStrpAlt format = 0x1f21
 )
 
 // A Tag is the classification (the type) of an Entry.

src/pkg/debug/dwarf/entry.go

@@ -241,10 +241,10 @@ func (b *buf) entry(atab abbrevTable, ubase Offset) *Entry {
 		// lineptr, loclistptr, macptr, rangelistptr
 		// New in DWARF 4, but clang can generate them with -gdwarf-2.
 		// Section reference, replacing use of formData4 and formData8.
-		case formSecOffset:
+		case formSecOffset, formGnuRefAlt, formGnuStrpAlt:
 			is64, known := b.format.dwarf64()
 			if !known {
-				b.error("unknown size for DW_FORM_sec_offset")
+				b.error("unknown size for form 0x" + strconv.FormatInt(int64(fmt), 16))
 			} else if is64 {
 				val = int64(b.uint64())
 			} else {