libstdc++: Fix std::numeric_limits<__float128>::max_digits10 [PR121374]

When I added this explicit specialization in r14-1433-gf150a084e25eaa I
used the wrong value for the number of mantissa digits (I used 112
instead of 113). Then when I refactored it in r14-1582-g6261d10521f9fd I
used the value calculated from the incorrect value (35 instead of 36).

libstdc++-v3/ChangeLog:

	PR libstdc++/121374
	* include/std/limits (numeric_limits<__float128>::max_digits10):
	Fix value.
	* testsuite/18_support/numeric_limits/128bit.cc: Check value.

(cherry picked from commit cf88ed5bf20c86ca38da19358ff79a34adb4d0b5)
2 files changed