debug/pe: use correct pe64 test in ImportedSymbols

64-bit PE is set by the optional header magic number,
not by the machine field.

Fixes #54250

Change-Id: I4d01bc08553780d5470e63f0b17bbd968323e5d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/421394
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/debug/pe/file.go

@@ -322,7 +322,7 @@ func (f *File) ImportedSymbols() ([]string, error) {
 		return nil, nil
 	}
 
-	pe64 := f.Machine == IMAGE_FILE_MACHINE_AMD64 || f.Machine == IMAGE_FILE_MACHINE_ARM64
+	_, pe64 := f.OptionalHeader.(*OptionalHeader64)
 
 	// grab the number of data directory entries
 	var dd_length uint32