debug/elf: Expose entry point from Header in File struct.

Fixes #3470.

R=rsc, golang-dev
CC=golang-dev
https://golang.org/cl/6195074

src/pkg/debug/elf/file.go

@@ -31,6 +31,7 @@ type FileHeader struct {
 	ByteOrder  binary.ByteOrder
 	Type       Type
 	Machine    Machine
+	Entry      uint64
 }
 
 // A File represents an open ELF file.
@@ -240,6 +241,7 @@ func NewFile(r io.ReaderAt) (*File, error) {
 		}
 		f.Type = Type(hdr.Type)
 		f.Machine = Machine(hdr.Machine)
+		f.Entry = uint64(hdr.Entry)
 		if v := Version(hdr.Version); v != f.Version {
 			return nil, &FormatError{0, "mismatched ELF version", v}
 		}
@@ -258,6 +260,7 @@ func NewFile(r io.ReaderAt) (*File, error) {
 		}
 		f.Type = Type(hdr.Type)
 		f.Machine = Machine(hdr.Machine)
+		f.Entry = uint64(hdr.Entry)
 		if v := Version(hdr.Version); v != f.Version {
 			return nil, &FormatError{0, "mismatched ELF version", v}
 		}

src/pkg/debug/elf/file_test.go

@@ -24,7 +24,7 @@ type fileTest struct {
 var fileTests = []fileTest{
 	{
 		"testdata/gcc-386-freebsd-exec",
-		FileHeader{ELFCLASS32, ELFDATA2LSB, EV_CURRENT, ELFOSABI_FREEBSD, 0, binary.LittleEndian, ET_EXEC, EM_386},
+		FileHeader{ELFCLASS32, ELFDATA2LSB, EV_CURRENT, ELFOSABI_FREEBSD, 0, binary.LittleEndian, ET_EXEC, EM_386, 0x80483cc},
 		[]SectionHeader{
 			{"", SHT_NULL, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
 			{".interp", SHT_PROGBITS, SHF_ALLOC, 0x80480d4, 0xd4, 0x15, 0x0, 0x0, 0x1, 0x0},
@@ -67,7 +67,7 @@ var fileTests = []fileTest{
 	},
 	{
 		"testdata/gcc-amd64-linux-exec",
-		FileHeader{ELFCLASS64, ELFDATA2LSB, EV_CURRENT, ELFOSABI_NONE, 0, binary.LittleEndian, ET_EXEC, EM_X86_64},
+		FileHeader{ELFCLASS64, ELFDATA2LSB, EV_CURRENT, ELFOSABI_NONE, 0, binary.LittleEndian, ET_EXEC, EM_X86_64, 0x4003e0},
 		[]SectionHeader{
 			{"", SHT_NULL, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
 			{".interp", SHT_PROGBITS, SHF_ALLOC, 0x400200, 0x200, 0x1c, 0x0, 0x0, 0x1, 0x0},