stand: Initial kboot support on amd64

Get amd64 compiling. However, the current kboot supports an old way of
enumerating memory and the new way needs to be incorporated as well. The
powerpc folks could use either, it seems and newer powerpc platforms
need some changes for kboot to work anyway.

This commit includes the linker script, trampoline code to start the new
kernel, Linux system calls and the necessary configuration glue needed
to build the binaries.

This includes a quick hack to get multiboot support, but we need to
really share these defines. The multiiboot2.h is the minimum needed to
build. We have multiboot information in three places now, so a
refactoring is in order.

This should be considered, at best, preliminary and experimental for
anybody wishing to try it out.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D35100

(cherry picked from commit 7df08a14e3fda5d0e7f3aef523ffcc7b3fae248d)