Eliminate p_elf_machine from struct proc.

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.

Note to MFC: D30918, KBI

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D30926
MFC after: 2 weeks