arch/x86: complete 64-bit linker script

The previous linker script was barebones and non-standard. It is
replaced with a script conforms to the rest of the Zephyr arches,
utilizing include/linker headers and standard macros.

link-tool-gcc.h is updated to account for the "i386:x86-64" arch and
the generation of 64-bit ELF binaries.

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