Add support for ELF64

ELF32 was assumed so far, and a few hard coded constants were used. In this
patch, we avoid hardcoding and header size which are different for ELF32 and
ELF64.

Use elfio.get_class() to set the output file ELF class so that it matches the
input file ELF class.

signlk needs the section size header to compute the offset where the data needs
to be inserted, so a new file is created which contains the section size which
is different on ELF32 and ELF64.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>