arch/x86: initial Intel64 bootstrap framework

This patch adds basic build infrastructure, definitions, a linker
script, etc. to use the Zephyr and 0.10.1 SDK to build a 64-bit
ELF binary suitable for use with GRUB to minimally bootstrap an
Apollo Lake (e.g., UpSquared) board. The resulting binary can hardly
be called a Zephyr kernel as it is lacking most of the glue logic,
but it is a starting point to flesh those out in the x86 tree.

The "kernel" builds with a few harmless warnings, both with GCC from
the Zephyr SDK and with ICC (which is currently being worked on in
a separate branch). These warnings are either related to pointer size
differences (since this is an LP64 build) and/or dummy functions
that will be replaced with working versions shortly.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>