The upcoming release of gcc 10 has 64-bit long double as an configurable option. In order to prepare fp64lib for that, a new macro was introduced GCC_ENTRY to provide compatible naming with gcc 10. Furthermore, the following functions were added for improved compatibility: fp64_cotan(x), fp64_exp10(x), fp64_pow10(x) and fp64_log2.
For further instruction on how to use the long double support, see this page.
Except for the new functions, the implicit long double support is currently not availabe inside the Arduino IDE, as gcc 10 is still in the testing/ experimental phase and not yet offically released.