Assembler fails if using input/output operands

Using the example file [func_calls.py](https://github.com/eliben/pycparser/blob/master/examples/func_calls.py) from **pycparser**, with the only modification of instantiating a GnuCParser:
```
def show_func_calls(filename, funcname):
gnu_parser = GnuCParser()
ast = parse_file(filename, use_cpp=False, parser=gnu_parser)
v = FuncCallVisitor(funcname)
v.visit(ast)
```

Basic "asm" expressions are parsed correctly, such as
`asm ("foo" ::: "bar");`
or
`__asm__ ("foo" ::: "bar", "bar");`

However, any asm expression with i/o operands will fail with the error: **AttributeError: 'Constant' object has no attribute 'name'**.

This is the test file I'm using:
```
#include
void main(void)
{
// Sample: asm volatile("" ::: "memory")
// => WORKS
asm ("foo" ::: "bar");

// Sample: __asm__ __volatile__("sub pc, pc, -4" ::: "memory", "cc")
// => WORKS
__asm__ ("foo" ::: "bar", "bar");

// Sample: __asm__ ("brev\t%0" : "+r" (__value) : : "cc")
// => ERROR
__asm__ ("foo" : "bar" (5) : : "bar");

// Sample: __asm__ __volatile__ ("ssrf\t%0" : : "i" (OFFSET))
// => ERROR
__asm__ ("foo" : : "bar" (5));

// Sample: __asm__ ("max\t%0, %1, %2" : "=r" (__value) : "r" (__arg_a), "r" (__arg_b))
// => ERROR
__asm__ ("foo" : "bar" (5) : "bar" (6), "baz" (7));
}
```

The samples are lines of code taken from a real application, I don't have much knowledge about the syntax of these GCC extension but these lines compile and work correctly with GCC 4.4.7 so I assume they are valid variations of the asm function.