Copy the e_machine and e_flags fields from the binary into an ELF core dump.

In the kernel, cache the machine and flags fields from ELF header to use in
the ELF header of a core dump. For gcore, the copy these fields over from
the ELF header in the binary.

This matters for platforms which encode ABI information in the flags field
(such as o32 vs n32 on MIPS).

Reviewed by: kib
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D9392