[gdb] Fix a compiler warning that shows up only when assertions are enabled

gcc should (I think) be warning about this anyway, but seems to do so
only when assertions are enabled for this object.

src/arch/i386/core/gdbmach.c

 }
 
 static void gdbmach_commit_hwbp ( struct hwbp *bp ) {
-	int regnum = bp - hwbps;
+	unsigned int regnum = bp - hwbps;
 
 	/* Set breakpoint address */
-	assert ( regnum >= 0 && regnum < sizeof hwbps / sizeof hwbps [ 0 ] );
+	assert ( regnum < ( sizeof hwbps / sizeof hwbps [ 0 ] ) );
 	switch ( regnum ) {
 		case 0:
 			__asm__ __volatile__ ( "movl %0, %%dr0\n" : : "r" ( bp->addr ) );