libstdc++-8-dev-armel-cross 8.4.0-3ubuntu1cross1 (riscv64 binary) in ubuntu focal

 This package contains the headers and static library files necessary for
 building C++ programs which use libstdc++.
 .
 libstdc++-v3 is a complete rewrite from the previous libstdc++-v2, which
 was included up to g++-2.95. The first version of libstdc++-v3 appeared
 in g++-3.0.
 .
 This package contains files for armel architecture, for use in cross-compile
 environment.