libstdc++: Fix std::from_chars to ignore leading zeros in base 2

libstdc++: Fix std::from_chars to ignore leading zeros in base 2

The parser for binary numbers returned an error if the entire string
contains more digits than the result type. Leading zeros should be
ignored.

libstdc++-v3/ChangeLog:

	* include/std/charconv (__from_chars_binary): Ignore leading zeros.
	* testsuite/20_util/from_chars/1.cc: Check "0x1" for all bases,
	not just 10 and 16.
	* testsuite/20_util/from_chars/3.cc: New test.