Add an elf not so kgdb detects the kernel as a FreeBSD elf file. The
ELFNOTE macro is based on one from the FreeBSD/ARM Xen tree [1].

Obtained from:	Julien Grall <julien.grall AT linaro.org> [1]


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273251 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f