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

(cherry picked from commit 945accf502db0dd44522489857f461e29ef17d30)