libelf: Use offsetof to get field of unaligned

libelf: Use offsetof to get field of unaligned

gcc undefined sanitizer flags:

elf_begin.c:230:18: runtime error: member access within misaligned
address 0xf796400a for type 'struct Elf64_Shdr', which requires 4 byte
alignment struct.

We aren't actually accessing the field member of the struct, but are
taking the address of it. Which the compiler can take as a hint that
the address is correctly aligned. But we can do the same by adding
the field offsetof to the base address. Which doesn't trigger a
runtime error.

Signed-off-by: Mark Wielaard <mark@klomp.org>