libm -fno-builtin

[jon@beniston.com]

>Using -fno-builtin-memcpy doesn't prevent GCC from replacing a loop 
>with a call to memcpy (likewise for memset). As Joesph said before, 
>-fno-tree-loop-distribute-patterns should be used for that. However, 
>this probably it isn't needed, as this only gets enabled at -O3 anyway, 
>and Newlib compiles with O2.
>
>Note that -fno-builtin does prevent this, oddly, but obviously 
>we don't want to use that.
>
>Perhaps GCC should be a little bit more clever, and not try to use this 
>optimisation if the function called matches the name of the function being
compiled.

Ah, there's already a PR for that:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56888

Cheers,
Jon