__ieee754_lgamma_r reentrency with nano.specs

Hi,

I have an issue when using __ieee754_lgamma_r (through lgamma) together
with nano.specs. I think libm is compiled referencing the full struct
_reent and thus a call to lgamma overwrites memory out of the small struct
_reent used by libc_nano.

I'm using newlib on an stm32 from debian sid libnewlib-arm-none-eabi
package, but I also have the problem with ARM provided toolchain
(gcc-arm-none-eabi-8-2019-q3-update).

I don't know if it is the right place to submit this kind of problem. If it
is, I can provide more information or even some code showing the issue if
needed.

Damien