elf_common.h: define EF_ARM_EABI_VERSION

The Linux distributions have had the EF_ARM_EABI_VERSION macro for a
while now. The kernel of arm* targets build depends on it being in the
host's elf.h environment. Add it here so it gets pulled in so there's
one fewer hack required to build a Linux kernel on a FreeBSD host.

Sponsored by: Netflix
Reviewed by: jrtc27, kib, jhb
Differential Revision: https://reviews.freebsd.org/D38139