<cmath> not compatible with -mdouble=64

[sprintersb]

As it seems, this library is not compatible with -mdouble=64, which is enabled in the default conrigurations since v10, see

• https://gcc.gnu.org/gcc-10/changes.html#avr
• https://gcc.gnu.org/onlinedocs/gcc/AVR-Options.html#index-mdouble

[ckormanyos]

As it seems, this library is not compatible with -mdouble=64

Personally I haven't done much with 64-bit double in association with this port of <cmath>.

Hi @sprintersb to move forward, could you please some examples or experiences exhibiting incompatibilities? We might need some rewites or extensions here.