Fix enum flag with Python 3

Fix enum flag with Python 3

Using Python 3.5 (I assume it's the same with 3.4 and lower, but I didn't
test), I see this:

  print (enum flag_enum) (FLAG_1)^M
  Python Exception <class 'TypeError'> %x format: an integer is required, not gdb.Value: ^M
  $7 = ^M
  (gdb) FAIL: gdb.python/py-pp-maint.exp: print FLAG_1

Apparently, this idiom, where v is a gdb.Value, was possible with Python 2,
but not with Python 3:

  '%x' % v

In Python 2, it would automatically get converted to an integer.  To solve
it, I simply added wrapped v in a call to int().

  '%x' % int(v)

In Python 2, the int type is implemented with a "long" in C, so on x86-32 it's
32-bits.  I was worried that doing int(v) would truncate the value and give
wrong results for enum values > 32-bits.  However, the int type != the int
function.  The int function does the right thing, selecting the right integer
type for the given value.  I tested with large enum values on x86-32 and
Python 2, and everything works as expected.

gdb/ChangeLog:

	* python/lib/gdb/printing.py (_EnumInstance.to_string): Explicitly
	convert gdb.Value to integer type using int().