debug/dwarf: formStrp uses a 64-bit value for 64-bit DWARF

    
    No test as the only system I know that uses 64-bit DWARF is AIX.
    
    Backport of https://golang.org/cl/84379, which will be in Go 1.11.
    Backporting now for AIX support in gccgo.
    
    Reviewed-on: https://go-review.googlesource.com/87296

From-SVN: r256474

gcc/go/gofrontend/MERGE

-19d94969c5202c07b3b166079b9f4ebbb52dfa6b
+1176dd2b53f2d2b826b599a126f3f9828283cec3
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/debug/dwarf/entry.go

@@ -461,7 +461,18 @@ func (b *buf) entry(atab abbrevTable, ubase Offset) *Entry {
 		case formString:
 			val = b.string()
 		case formStrp:
-			off := b.uint32() // offset into .debug_str
+			var off uint64 // offset into .debug_str
+			is64, known := b.format.dwarf64()
+			if !known {
+				b.error("unknown size for DW_FORM_strp")
+			} else if is64 {
+				off = b.uint64()
+			} else {
+				off = uint64(b.uint32())
+			}
+			if uint64(int(off)) != off {
+				b.error("DW_FORM_strp offset out of range")
+			}
 			if b.err != nil {
 				return nil
 			}