My intention was to create a uclibc based system (and a uclibc based toolchain) for my NSLU2. This was not obvious in the post because building the uclibc based toolchain resulted in this error:
[..]/var/tmp/portage/cross-armv5-softfloat-linux-uclibceabi/gcc-4.9.2/work/gcc-4.9.2/libsanitizer/sanitizer_common/sanitizer_platform_limits_posix.cc:67:21: fatal error: wordexp.h: No such file or directory
Makefile:416: recipe for target 'sanitizer_platform_limits_posix.lo' failed
make: *** [sanitizer_platform_limits_posix.lo] Error 1
make: *** Waiting for unfinished jobs....
So, yesterday, to not forget what I did, I tried building a glibc based toolchain and posted that.
Today, after looking at the offending error, it seems gcc 4.9.2 assumes wordexp.h is always available on non-Android platforms and Gentoo does not make that file availble when installing uclibc.
crossdev --g 4.8.3 armv5-softfloat-linux-uclibceabi
I hope this helps other people. Yes, I know I should report the issue to GCC/Gentoo after further investigation.