xtensa: add default definition for XCHAL_HAVE_DIV32

When variant FSF is set, XCHAL_HAVE_DIV32 is not defined. Add default
definition for that macro to prevent build warnings:

arch/xtensa/lib/divsi3.S:9:5: warning: "XCHAL_HAVE_DIV32" is not defined, evaluates to 0 [-Wundef]
    9 | #if XCHAL_HAVE_DIV32
arch/xtensa/lib/modsi3.S:9:5: warning: "XCHAL_HAVE_DIV32" is not defined, evaluates to 0 [-Wundef]
    9 | #if XCHAL_HAVE_DIV32

Fixes: 173d6681 ("xtensa: remove extra header files")
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: lore.kernel.org/r/202309150556.t0yCdv3g-lkp@intel.com

arch/xtensa/include/asm/core.h

@@ -6,6 +6,10 @@
 
 #include <variant/core.h>
 
+#ifndef XCHAL_HAVE_DIV32
+#define XCHAL_HAVE_DIV32 0
+#endif
+
 #ifndef XCHAL_HAVE_EXCLUSIVE
 #define XCHAL_HAVE_EXCLUSIVE 0
 #endif