[gdb] Add debug_{exp,val}

When debugging cc1 I heavily rely on simple one-parameter debug functions
that allow me to inspect a variable of a common type, like:
- debug_generic_expr
- debug_gimple_stmt
- debug_rtx
and I miss similar functions in gdb.

Add functions to dump variables of types 'value' and 'expression':
- debug_exp, and
- debug_val.

Tested on x86_64-linux, by breaking on varobj_create, and doing:
...
(gdb) call debug_exp (var->root->exp.get ())
&"Operation: OP_VAR_VALUE\n"
&" Block symbol:\n"
&"  Symbol: aaa\n"
&"  Block: 0x2d064f0\n"
(gdb)
...
and:
...
(gdb) call debug_val (value)
&"5"
(gdb)
...

gdbsupport/common-defs.h

@@ -191,6 +191,12 @@
 #define ATTRIBUTE_UNUSED_RESULT
 #endif
 
+#if (GCC_VERSION > 4000)
+#define ATTRIBUTE_USED __attribute__ ((__used__))
+#else
+#define ATTRIBUTE_USED
+#endif
+
 #include "libiberty.h"
 #include "pathmax.h"
 #include "gdb/signals.h"