The target C library (at least in mingw case, mingw-runtime) is built using CFLAGS from host's make.conf. But it is supposed to run on the target system, so it should be build using target CFLAGS (from /usr/${TARGET}/etc/portage/make.conf). The obvious workaround is to rebuild the package using ${TARGET}-emerge, then use crossdev to reemerge stage2 gcc, as it may contain code from it in its CRT files. Reproducible: Always
the cross-xxx packages are built in the host system, not the target. using the target make.conf won't work (not that it even exists when crossdev runs). at some point i might add a --target-cflags option to crossdev, but gcc/binutils need work first.
also, if you really want to tweak things in the mean time, use --lenv/--kenv to set custom CFLAGS for the library packages
