riscv64 cross-compile fails to link on Arch

[Dusten Sobotta]

[1/3] Linking CXX executable bin/toitc
FAILED: bin/toitc
: && /usr/bin/riscv64-linux-gnu-g++ '-fdebug-prefix-map=/home/dusten/git/toit-riscv=.' -Wall -Werror -ffunction-sections -fdata-sections -Wno-unused-command-line-argument -fno-exceptions -DMBEDTLS_SSL_IN_CONTENT_LEN=4608 -DMBEDTLS_SSL_OUT_CONTENT_LEN=3700 -DMBEDTLS_PLATFORM_MEMORY=1 -Os -fno-rtti -DDEPLOY src/compiler/CMakeFiles/toitc.dir/toitc.cc.o -o bin/toitc -static-libstdc++ lib/libtoit_core.a lib/libtoit_compiler.a lib/libmbedtls.a -lpthread -ldl lib/libyaml.a lib/libsemver.a lib/libmbedx509.a lib/libmbedcrypto.a && :
/usr/lib/gcc/riscv64-linux-gnu/11.1.0/../../../../riscv64-linux-gnu/bin/ld: cannot find -lstdc++
collect2: error: ld returned 1 exit status
[2/3] Linking CXX executable bin/toitvm
FAILED: bin/toitvm
: && /usr/bin/riscv64-linux-gnu-g++ '-fdebug-prefix-map=/home/dusten/git/toit-riscv=.' -Wall -Werror -ffunction-sections -fdata-sections -Wno-unused-command-line-argument -fno-exceptions -DMBEDTLS_SSL_IN_CONTENT_LEN=4608 -DMBEDTLS_SSL_OUT_CONTENT_LEN=3700 -DMBEDTLS_PLATFORM_MEMORY=1 -Os -fno-rtti -DDEPLOY src/CMakeFiles/toitvm.dir/toit.cc.o src/CMakeFiles/toitvm.dir/checksum.cc.o -o bin/toitvm -static-libstdc++ -Wl,--gc-sections -Wl,--whole-archive lib/libtoit_vm.a lib/libmbedtls.a -Wl,--no-whole-archive lib/libtoit_compiler.a -lpthread -ldl -static-libgcc -lSegFault lib/libmbedx509.a lib/libmbedcrypto.a lib/libyaml.a lib/libsemver.a && :
/usr/lib/gcc/riscv64-linux-gnu/11.1.0/../../../../riscv64-linux-gnu/bin/ld: cannot find -lstdc++
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.
make: *** [Makefile:38: build/riscv64/bin/toitvm] Error 1