crunchide: Correct 64-bit section header offset

For 64-bit binaries the Elf_Ehdr e_shoff is at offset 40, not 44.
Instead of using an incorrect hardcoded offset, let the compiler
figure it out for us with offsetof().

Differential Revision: https://reviews.freebsd.org/D1543
MFC after: 1 week
Sponsored by: The FreeBSD Foundation