I can't find any documentation whatsoever on how to target 32-bit with Red Hat Developer Toolset (version 9.0 in this case running on CentOS 7). The release notes mention:
Generation and manipulation of 32-bit binaries is also supported
It also ships the needed 32-bit libraries in:
/opt/rh/devtoolset-9/root/usr/lib/gcc/x86_64-redhat-linux/9/32/
However, building fails. Example trying to build a minimal int main() {} program:
$ scl enable devtoolset-9 'g++ -m32 tst.cpp'
/opt/rh/devtoolset-9/root/usr/libexec/gcc/x86_64-redhat-linux/9/ld: skipping incompatible /opt/rh/devtoolset-9/root/usr/lib/gcc/x86_64-redhat-linux/9/libstdc++_nonshared.a when searching for -lstdc++_nonshared
/opt/rh/devtoolset-9/root/usr/libexec/gcc/x86_64-redhat-linux/9/ld: cannot find -lstdc++_nonshared
collect2: error: ld returned 1 exit status
The library it fails to find actually exists though:
/opt/rh/devtoolset-9/root/usr/lib/gcc/x86_64-redhat-linux/9/32/libstdc++_nonshared.a
No amount of -L flags fixes it (and it would be the wrong solution anyway; the linker should not even be attempting to load 64-bit libraries in -m32 mode.)
What am I missing here?
I guess you didn't notice that /opt/rh/devtoolset-9/root/usr/lib/gcc/x86_64-redhat-linux/9/32/libstdc++_nonshared.a is quite likely a dangling symlink:
$ file /opt/rh/devtoolset-9/root/usr/lib/gcc/x86_64-redhat-linux/9/32/libstdc++_nonshared.a
/opt/rh/devtoolset-9/root/usr/lib/gcc/x86_64-redhat-linux/9/32/libstdc++_nonshared.a: broken symbolic link to `../../../i686-redhat-linux/9/libstdc++_nonshared.a'
$
The destination file /opt/rh/devtoolset-9/root/usr/lib/gcc/i686-redhat-linux/9/libstdc++_nonshared.a is however unfortunately not provided by any CentOS package (but it should be in devtoolset-9-libstdc++-devel.i686). Thus it's likely a CentOS-specific packaging mistake as RHEA-2019:4134 provides the questioned package devtoolset-9-libstdc++-devel-9.1.1-2.6.el7.i686.rpm for Red Hat Enterprise Linux 7 including the desired file (explicitly verified by yum install /opt/rh/devtoolset-9/root/usr/lib/gcc/i686-redhat-linux/9/libstdc++_nonshared.a on RHEL 7).