debug/pe: support PE files which contain no symbol table (if NumberOfSymbols is equal to 0 in the IMAGE_FILE_HEADER structure).

No longer assume that e_lfanew (in the IMAGE_DOS_HEADER strcuture) is always one byte. It is now regarded as a 4 byte uint32.

Fixes #4177.

R=golang-dev, alex.brainman, dave, minux.ma
CC=golang-dev
https://golang.org/cl/6587048