Expand the elf brandelf infrastructure to give access to the whole ELF
header (Elf_Ehdr) to determine if a particular interpretor wants to
accept it or not. Use this mechanism to filter EABI arm on OABI arm
kernels, and vice versa. This method could also be used to implement
OABI on EABI arm kernels, if desired, or to allow a single mips kernel
to run o32, n32 and n64 binaries.

Differential Revision: https://reviews.freebsd.org/D609
/usr/libexec/ld.so.1 never was a thing on FreeBSD/arm. This was the
FreeBSD 3.x and 4.x run time linker. FreeBSD/arm's first release was
5.0. Retire this long-dead code.