[Bug libstdc++/90046] [9 Regression] fails to build a epiphany-elf cross toolchain with C++ enabled

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90046

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I can't even figure out how to configure for this target:

checking stdbool.h usability... no
checking stdbool.h presence... yes
configure: WARNING: stdbool.h: present but cannot be compiled
configure: WARNING: stdbool.h:     check for missing prerequisite headers?
configure: WARNING: stdbool.h: see the Autoconf documentation
configure: WARNING: stdbool.h:     section "Present But Cannot Be Compiled"
configure: WARNING: stdbool.h: proceeding with the compiler's result
checking for stdbool.h... no
checking stdalign.h usability... no
checking stdalign.h presence... yes
configure: WARNING: stdalign.h: present but cannot be compiled
configure: WARNING: stdalign.h:     check for missing prerequisite headers?
configure: WARNING: stdalign.h: see the Autoconf documentation
configure: WARNING: stdalign.h:     section "Present But Cannot Be Compiled"
configure: WARNING: stdalign.h: proceeding with the compiler's result
checking for stdalign.h... no
checking for the value of EOF... configure: error: computing EOF failed
make: *** [Makefile:10842: configure-target-libstdc++-v3] Error 1