kernel/arch: optimize memory use of some thread fields

Some thread fields were 32-bit wide, when they are not even close to
using that full range of values. They are instead changed to 8-bit fields.

- prio can fit in one byte, limiting the priorities range to -128 to 127

- recursive scheduler locking can be limited to 255; a rollover results
  most probably from a logic error

- flags are split into execution flags and thread states; 8 bits is
  enough for each of them currently, with at worst two states and four
  flags to spare (on x86, on other archs, there are six flags to spare)

Doing this saves 8 bytes per stack. It also sets up an incoming
enhancement when checking if the current thread is preemptible on
interrupt exit.

Change-Id: Ieb5321a5b99f99173b0605dd4a193c3bc7ddabf4
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>

arch/arm/core/exc_exit.S

@@ -84,8 +84,13 @@ SECTION_SUBSEC_FUNC(TEXT, _HandlerModeExit, _ExcExit)
     ldr r0, =_kernel
 
     ldr r1, [r0, #_kernel_offset_to_current]
-    ldr r2, [r1, #_thread_offset_to_prio]
-    ldr r3, [r1, #_thread_offset_to_sched_locked]
+#ifdef CONFIG_CPU_CORTEX_M0_M0PLUS
+    movs r3, #_thread_offset_to_prio
+    ldrsb r2, [r1, r3]
+#else
+    ldrsb r2, [r1, #_thread_offset_to_prio]
+#endif
+    ldrb r3, [r1, #_thread_offset_to_sched_locked]
 
     /* coop thread ? do not schedule */
     cmp r2, #0