libm -fno-builtin

[Wilco Dijkstra]

> Hello Jon!
>
> So the final opinion is that it is safe to use the new `--disable-
> newlib-fno-builtin` option with ARM MCU targets, right? (;

It is always safe to use -fbuiltin - it's basic code generation.  As discussed
-fbuiltin is used in all other C libraries without issues and is already the default in
libm/common.

So I don't understand why it is not the default in all of newlib and why it needs a 
configure option to enable it. Do we really want to force every user of newlib to use 
a long set of configure options to enable basic codegeneration features?

Wilco