util/grub-module-verifierXX: Add function to calculate section headers

util/grub-module-verifierXX: Add function to calculate section headers

Added the function get_shdr() which returns the section header at a given index
parameter passed into this function. This helps traverse the section header
table and reduces repeated calls to lengthy equations used to obtain section
headers.

Note that it may look as though the argument *arch isn't being used, it's
actually required in order to use the macros grub_target_to_host*(x) which are
unwinded to grub_target_to_host*_real(arch, (x)) based on defines earlier in the
file.

Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>