Support higher baud rates when they are defined

On Linux at least, baud rate codes are defined up to B4000000. Allow the
user to select them if they are present in the system headers.

Change-Id: I393ff32e4a4b6127bdf97e3306ad5b6ebf7c934e

gdb/ser-unix.c

@@ -327,6 +327,72 @@ baudtab[] =
     460800, B460800
   }
   ,
+#endif
+#ifdef B500000
+  {
+    500000, B500000
+  }
+  ,
+#endif
+#ifdef B576000
+  {
+    576000, B576000
+  }
+  ,
+#endif
+#ifdef B921600
+  {
+    921600, B921600
+  }
+  ,
+#endif
+#ifdef B1000000
+  {
+    1000000, B1000000
+  }
+  ,
+#endif
+#ifdef B1152000
+  {
+    1152000, B1152000
+  }
+  ,
+#endif
+#ifdef B1500000
+  {
+    1500000, B1500000
+  }
+  ,
+#endif
+#ifdef B2000000
+  {
+    2000000, B2000000
+  }
+  ,
+#endif
+#ifdef B2500000
+  {
+    2500000, B2500000
+  }
+  ,
+#endif
+#ifdef B3000000
+  {
+    3000000, B3000000
+  }
+  ,
+#endif
+#ifdef B3500000
+  {
+    3500000, B3500000
+  }
+  ,
+#endif
+#ifdef B4000000
+  {
+    4000000, B4000000
+  }
+  ,
 #endif
   {
     -1, -1