This adds simple ELF test binaries generated by gcc and clang and
compares the line tables returned by the line table reader against
tables based on the output of readelf.

The binaries were generated with
    # gcc --version | head -n1
    gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2
    # gcc -g -o line-gcc.elf line*.c

    # clang --version | head -n1
    Ubuntu clang version 3.4-1ubuntu3 (tags/RELEASE_34/final) (based on LLVM 3.4)
    # clang -g -o line-clang.elf line*.c

Change-Id: Id210fdc1d007ac9719e8f5dc845f2b94eed12234
Reviewed-on: https://go-review.googlesource.com/7070
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Rob Pike <r@golang.org>