xtensa: make sure bFLT stack is 16 byte aligned

Xtensa ABI requires stack alignment to be at least 16. In noMMU
configuration ARCH_SLAB_MINALIGN is used to align stack. Make it at
least 16.

This fixes the following runtime error in noMMU configuration, caused by
interaction between insufficiently aligned stack and alloca function,
that results in corruption of on-stack variable in the libc function
glob:

 Caught unhandled exception in 'sh' (pid = 47, pc = 0x02d05d65)
  - should not happen
  EXCCAUSE is 15

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/processor.h

@@ -23,7 +23,11 @@
 # error Linux requires the Xtensa Windowed Registers Option.
 #endif
 
-#define ARCH_SLAB_MINALIGN	XCHAL_DATA_WIDTH
+/* Xtensa ABI requires stack alignment to be at least 16 */
+
+#define STACK_ALIGN (XCHAL_DATA_WIDTH > 16 ? XCHAL_DATA_WIDTH : 16)
+
+#define ARCH_SLAB_MINALIGN STACK_ALIGN
 
 /*
  * User space process size: 1 GB.