x86: consolidate x86_64 architecture, SoC and boards

There are two set of code supporting x86_64: x86_64 using x32 ABI,
and x86 long mode, and this consolidates both into one x86_64
architecture and SoC supporting truly 64-bit mode.

() Removes the x86_64:x32 architecture and SoC, and replaces
   them with the existing x86 long mode arch and SoC.
() Replace qemu_x86_64 with qemu_x86_long as qemu_x86_64.
() Updates samples and tests to remove reference to
   qemu_x86_long.
() Renames CONFIG_X86_LONGMODE to CONFIG_X86_64.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>