On a x86_64 linux box I successfully built a gcc RC-20150418 toolchain including binutils-2.25 and avr-libc r2473(svn).
I noticed some warnings in the logs: ../../../source/gcc-5.0.1-RC-20150418/gcc/vec.h:1048: warning: invalid access to non-static data member âvec<ipa_polymorphic_call_context, va_heap, vl_embed>::m_vecdataâ of NULL object ../../../source/gcc-5.0.1-RC-20150418/gcc/vec.h:1048: warning: (perhaps the âoffsetofâ macro was used incorrectly) but the build completed successfully One small problem I found is that earlier snapshots (RC-20150405 I think) built OK without texinfo, but this RC build fails without it. I just installed texinfo instead of trying to fix the configure script. I compiled a small C program with -mmcu=attiny84,-Os and got a good binary. I also tried -Os -ftlo, and got a good binary. Unfortunately, despite the announced optimization improvements for lto and ipa, I'm seeing slightly larger binaries than 4.9.2. Hopefully the next 5.x release will surpass 4.9.2. Here's the build scripts I used if anyone wants to repeat it: https://github.com/nerdralph/nerdralph/tree/master/avr/make-avr-gcc -Ralph