x86: x86_64: fix tls setup in early boot

During early boot in assembly, the function parameter to
z_x86_early_tls_update_gdt() should be the pointer to
the interrupt stack. However, what was passed instead
was the pointer to the x86_cpuboot struct. So fix it to
actually pass the stack pointer (which is stashed inside
the x86_cpuboot struct).

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