Preserve sign when converting gdb.Value to Python int

Preserve sign when converting gdb.Value to Python int

PR python/20126 points out that sometimes the conversion of a
gdb.Value can result in a negative Python integer.  This happens
because valpy_int does not examine the signedness of the value's type.

gdb/ChangeLog
2018-09-23  Tom Tromey  <tom@tromey.com>

	PR python/20126:
	* python/py-value.c (valpy_int): Respect type sign.

gdb/testsuite/ChangeLog
2018-09-23  Tom Tromey  <tom@tromey.com>

	PR python/20126:
	* gdb.python/py-value.exp (test_value_numeric_ops): Add
	signed-ness conversion tests.