[bug #57071] Fix math.h and function names that block 64-bit double

[Georg-Johann Lay]

Follow-up Comment #9, bug #57071 (project avr-libc):

The GCC feature is upstream: https://gcc.gnu.org/r277908

The new configure options are:


--with-double={32|64|32,64|64,32}
--with-long-double={32|64|32,64|64,32|double}


For a documentation, see

https://gcc.gnu.org/install/configure.html#avr

Only 14 of the 20 combinations are valid, resulting in 12 different
configurations total.  Depending on the configuration, there will be up to 57
= 3*19 multilib variants.

The new compiler options are:


-mdouble={32|64}
-mlong-double={32|64}


For a documentation, see

https://gcc.gnu.org/onlinedocs/gcc/AVR-Options.html#index-mdouble

To display the mapping between option sets and multilib variants, use


avr-gcc -print-multi-lib


To show which multilib variant is actually picked for a specific set of
options, use


avr-gcc -print-multi-directory <options>


To factor out different floating-point layouts in a target program, you can
use -- like always -- the built-in defines


__SIZEOF_DOUBLE__
__SIZEOF_LONG_DOUBLE__


The libgcc build just copies the double64 and long-double64 multilib variants
from the vanilla one so that the build times won't go through the roof --
these variants would not contain any differences to the vanilla version,
anyway.

There is also the (undocumented, avr) configure option --with-fixed-point=no
which skips the build of the thousands of libgcc fixed-point modules.  You can
use this during development for sane build times.

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