Rename i386's Linux ELF to Linux ELF32

This is what amd64 calls the i386 Linux ABI in order to distinguish it
from the amd64 Linux ABI, and matches the nomenclature used for the
FreeBSD ABIs where they always have the size suffix in the name.

Reviewed by: trasz
Differential Revision: https://reviews.freebsd.org/D27647