boards/x86: add Intel64 (64-bit long-mode) QEMU target board

Add qemu_x86_long board (with CONFIG_X86_LONGMODE=y) for testing.

This requires adding support to soc/ia32 for long mode (trivial),
and adding a quick 64- to 32-bit ELF conversion before invoking
QEMU, which apparently doesn't like 64-bit kernel files.

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