arch/x86: (Intel64) define TSS in C, not assembly

Declare the 64-bit TSS as a struct, and define the instance in C.
Add a data segment selector that overlaps the TSS and keep that
loaded in GS so we can access the TSS via a segment-override prefix.

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

arch/x86/core/intel64/tss.c

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2019 Intel Corporation
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <kernel.h>
+#include <kernel_arch_data.h>
+#include <kernel_arch_func.h>
+
+extern u8_t _exception_stack[];
+
+Z_GENERIC_SECTION(.tss)
+struct x86_tss64 tss0 = {
+	.ist1 = (u64_t) _interrupt_stack + CONFIG_ISR_STACK_SIZE,
+	.ist7 = (u64_t) _exception_stack + CONFIG_EXCEPTION_STACK_SIZE,
+
+	.iomapb = 0xFFFF	/* no I/O access bitmap */
+};