Eliminate p_elf_machine from struct proc.

[dchagin]

Summary

Instead of p_elf_machine use machine member of the Elf_Brandinfo which is now
cached in the struct proc at p_elf_brandinfo member.

As p_elf_flags alignment is 8 bytes here is a hole, should I remove p_reserved?

[kib]

sys/sys/proc.h

702–703

We do not leave the cruft in system structures (in HEAD), even if it is padding.

[markj]

+1 to kib's comment.