Generate IMAGE_REL_AMD64_ADDR32NB relocations for SEH
 data structures.

The Win64 EH data structures must be of type IMAGE_REL_AMD64_ADDR32NB
instead of IMAGE_REL_AMD64_ADDR32. This is easiely achieved by adding
the VK_COFF_IMGREL32 modifier to the symbol reference.
Change also references to start and end of the SEH range of a function
as offsets to start of the function.

Reviewed by Jim Grosbach, Charles Davis and Nico Rieck.