kernel: expose struct k_thread implementation

Historically, space for struct k_thread was always carved out of the
thread's stack region. However, we want more control on where this data
will reside; in memory protection scenarios the stack may only be used
for actual stack data and nothing else.

On some platforms (particularly ARM), including kernel_arch_data.h from
the toplevel kernel.h exposes intractable circular dependency issues.
We create a new per-arch header "kernel_arch_thread.h" with very limited
scope; it only defines the three data structures necessary to instantiate
the arch-specific bits of a struct k_thread.

Change-Id: I3a55b4ed4270512e58cf671f327bb033ad7f4a4f
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

arch/arm/include/kernel_arch_thread.h

@@ -0,0 +1,103 @@
+/*
+ * Copyright (c) 2017 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @file
+ * @brief Per-arch thread definition
+ *
+ * This file contains defintions for
+ *
+ *  struct _thread_arch
+ *  struct _callee_saved
+ *  struct _caller_saved
+ *
+ * necessary to instantiate instances of struct k_thread.
+ */
+
+#ifndef _kernel_arch_thread_h_
+#define _kernel_arch_thread_h_
+
+#ifndef _ASMLANGUAGE
+#include <zephyr/types.h>
+
+struct _caller_saved {
+	/*
+	 * Unused for Cortex-M, which automatically saves the necessary
+	 * registers in its exception stack frame.
+	 *
+	 * For Cortex-A, this may be:
+	 *
+	 * u32_t a1;    // r0
+	 * u32_t a2;    // r1
+	 * u32_t a3;    // r2
+	 * u32_t a4;    // r3
+	 * u32_t ip;    // r12
+	 * u32_t lr;    // r14
+	 * u32_t pc;    // r15
+	 * u32_t xpsr;
+	 */
+};
+
+typedef struct _caller_saved _caller_saved_t;
+
+struct _callee_saved {
+	u32_t v1;  /* r4 */
+	u32_t v2;  /* r5 */
+	u32_t v3;  /* r6 */
+	u32_t v4;  /* r7 */
+	u32_t v5;  /* r8 */
+	u32_t v6;  /* r9 */
+	u32_t v7;  /* r10 */
+	u32_t v8;  /* r11 */
+	u32_t psp; /* r13 */
+};
+
+typedef struct _callee_saved _callee_saved_t;
+
+#ifdef CONFIG_FLOAT
+struct _preempt_float {
+	float  s16;
+	float  s17;
+	float  s18;
+	float  s19;
+	float  s20;
+	float  s21;
+	float  s22;
+	float  s23;
+	float  s24;
+	float  s25;
+	float  s26;
+	float  s27;
+	float  s28;
+	float  s29;
+	float  s30;
+	float  s31;
+};
+#endif
+
+struct _thread_arch {
+
+	/* interrupt locking key */
+	u32_t basepri;
+
+	/* r0 in stack frame cannot be written to reliably */
+	u32_t swap_return_value;
+
+#ifdef CONFIG_FLOAT
+	/*
+	 * No cooperative floating point register set structure exists for
+	 * the Cortex-M as it automatically saves the necessary registers
+	 * in its exception stack frame.
+	 */
+	struct _preempt_float  preempt_float;
+#endif
+};
+
+typedef struct _thread_arch _thread_arch_t;
+
+#endif /* _ASMLANGUAGE */
+
+#endif /* _kernel_arch_thread__h_ */