Fix lib/msun/tests/csqrt_test on platforms with 128-bit long double

If long double has more than 64 mantissa bits, using uint64_t to hold the
mantissa bits will truncate the value and result in test failures. To fix
this problem use uint128_t since all platforms that have
LDBL_MANT_DIG__ > 64 also have compiler support for 128-bit integers.

Reviewed By: rlibby
Differential Revision: https://reviews.freebsd.org/D29076

(cherry picked from commit ce88eb476b86cac63cef7466bd71f14b611ab03a)