xtensa: Add Kconfig for asm2 layer

The asm2 layer will build alongside the traditional assembly, but the
reverse is not true.  Add a CONFIG_XTENSA_ASM2 to force its use at
runtime and disable the older code.

Note that the older assembly had an initialization function that is
properly part of the timer driver.  Move a C equivalent into the timer
driver itself for now to prevent a build breakage.  Long term we need
to clean that driver up in a bunch of other ways.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

arch/xtensa/Kconfig

@@ -93,6 +93,18 @@ config XTENSA_OMIT_HIGH_INTERRUPTS
 	code.  Default is "n" for legacy compatibility. Consider
 	changing to "y" in the future.
 
+config XTENSA_ASM2
+	bool
+	prompt "New-sytle Xtensa context switch & interrupt layer"
+	default n
+	select USE_SWITCH
+	help
+	This selects a new implementation of context switching and
+	interrupt handling.  Advantages are a much lower interrupt
+	overhead and smaller code size, and this scheme is required
+	for SMP.  Assumes/requires hardware that implements the
+	register window xtension, however.
+
 config TOOLCHAIN_VARIANT
 	string
 	default RG-2016.4-linux

arch/xtensa/core/CMakeLists.txt

@@ -2,19 +2,22 @@ zephyr_cc_option(-mlongcalls)
 zephyr_sources(
 	cpu_idle.c
 	fatal.c
-	irq_manage.c
-	swap.S
-	thread.c
-	xtensa_context.S
-	xtensa_intr_asm.S
-	xtensa_intr.c
-	xtensa_vectors.S
-	xt_zephyr.S
 	window_vectors.S
 	xtensa-asm2-util.S
 	xtensa-asm2.c
 	)
 
+zephyr_sources_ifndef(CONFIG_XTENSA_ASM2
+		      xtensa_intr.c
+		      irq_manage.c
+		      swap.S
+		      thread.c
+		      xtensa_context.S
+		      xtensa_intr_asm.S
+		      xtensa_vectors.S
+		      xt_zephyr.S
+		     )
+
 zephyr_sources_ifndef(CONFIG_ATOMIC_OPERATIONS_C atomic.S)
 zephyr_sources_ifdef(CONFIG_XTENSA_USE_CORE_CRT1
 	crt1.S