cris: arch-v10: kgdb: Add '__used' for static variable is_dyn_brkp

Within one C file, current gcc can optimize the global static variables
according to the C code, but it will skip assembly code -- it will pass
them to gas directly.

if the static variable is used between C code and assembly code in one C
file (e.g. is_dyn_brkp in kgdb.c), it needs '__used' to let gcc know it
should be still used, or gcc may remove it for optimization.

The related error in this case:

    LD      init/built-in.o
  arch/cris/arch-v10/kernel/built-in.o: In function `kgdb_handle_breakpoint':
  (.text+0x2aca): undefined reference to `is_dyn_brkp'
  arch/cris/arch-v10/kernel/built-in.o: In function `is_static':
  kgdb.c:(.text+0x2ada): undefined reference to `is_dyn_brkp'
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Jesper Nilsson <jespern@axis.com>

arch/cris/arch-v10/kernel/kgdb.c

@@ -352,7 +352,7 @@ char internal_stack[INTERNAL_STACK_SIZE];
    breakpoint to be handled. A static breakpoint uses the content of register
    BRP as it is whereas a dynamic breakpoint requires subtraction with 2
    in order to execute the instruction. The first breakpoint is static. */
-static unsigned char is_dyn_brkp = 0;
+static unsigned char __used is_dyn_brkp;
 
 /********************************* String library ****************************/
 /* Single-step over library functions creates trap loops. */