In trying to build an AVR toolchain recently, I discovered that crossdev does not modify the 'sanitize' USE flag at all. This makes sense when building for a regular target. However,it causes at least the AVR build of GCC to fail during the second stage. In the short term, I've worked around this by adding -sanitize to the gcc line in /etc/portage/package.use/cross-avr and marking the file immutable, and everything is building fine. Ideally, such a hack shouldn't be necessary.
This is a special case of the bug 370063. (It is recommended to work around by emerging with USE=-sanitize.)
*** This bug has been marked as a duplicate of bug 525910 ***
*** Bug 585220 has been marked as a duplicate of this bug. ***