sparc: Add kgdb support.

Current limitations:

1) On SMP single stepping has some fundamental issues,
   shared with other sw single-step architectures such
   as mips and arm.

2) On 32-bit sparc we don't support SMP kgdb yet.  That
   requires some reworking of the IPI mechanisms and
   infrastructure on that platform.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/asm-sparc/system.h

@@ -94,6 +94,8 @@ extern void fpsave(unsigned long *fpregs, unsigned long *fsr,
 	} while(0)
 #endif
 
+extern void flushw_all(void);
+
 /*
  * Flush windows so that the VM switch which follows
  * would not pull the stack from under us.