Add clog10 to builtins.def, round 2

[François-Xavier Coudert]

>>       * builtins.def: Add DEF_EXT_C99RES_BUILTIN to define builtins
>>       that C99 reserve for future use. Use it to define clog10, clog10f
>>       and clog10l.
> 
> Ok.

Commited.

Thanks,
FX