Stop print_hex from printing bits above the precision

Stop print_hex from printing bits above the precision

2017-10-26  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* wide-int-print.cc (print_hex): Loop based on extract_uhwi.
	Don't print any bits outside the precision of the value.
	* wide-int.cc (test_printing): Add some new tests.

From-SVN: r254109