sys-devel/gcc-9.2.0:9.2.0::core-kit USE="bootstrap cxx nls openmp pie ssp stack_clash_protection -ada (-altivec) -bootstrap-O3 -bootstrap-lean -bootstrap-lto -bootstrap-profiled -checking_all -checking_assert -checking_df -checking_extra -checking_fold -checking_gc -checking_gcac -checking_misc -checking_no -checking_release -checking_rtl -checking_rtlflag -checking_runtime -checking_tree -checking_valgrind -checking_yes -d -dev_extra_warnings -doc -fortran -generic_host -go -graphite (-jit) (-libssp) -link_now (-multilib) -objc -objc++ -objc-gc (-pch) (-sanitize) -ssp_all -stage1_checking_all -stage1_checking_assert -stage1_checking_df -stage1_checking_extra -stage1_checking_fold -stage1_checking_gc -stage1_checking_gcac -stage1_checking_misc -stage1_checking_no -stage1_checking_release -stage1_checking_rtl -stage1_checking_rtlflag -stage1_checking_runtime -stage1_checking_tree -stage1_checking_valgrind -stage1_checking_yes -test -vanilla (-vtv) (-abi_riscv_lp64%) (-abi_riscv_lp64d%)"

Making all in po
make[5]: Entering directory '/var/tmp/portage/sys-devel/gcc-9.2.0/work/objdir/x86_64-pc-linux-gnu/libstdc++-v3/po'
msgfmt -o de.mo ../../../../gcc-9.2.0/libstdc++-v3/po/de.po
msgfmt -o fr.mo ../../../../gcc-9.2.0/libstdc++-v3/po/fr.po
msgfmt: symbol lookup error: /usr/lib/libicuuc.so.67: undefined symbol: _ZSt15__once_callable, version GLIBCXX_3.4.11
make[5]: *** [Makefile:550: de.mo] Error 127
make[5]: *** Waiting for unfinished jobs....
msgfmt: symbol lookup error: /usr/lib/libicuuc.so.67: undefined symbol: _ZSt15__once_callable, version GLIBCXX_3.4.11
make[5]: *** [Makefile:550: fr.mo] Error 127