The background is we used extended buildtools SDK to provide gcc and other
build tools on host for a Yocto build, but recipes that uses gcc-multilib
and glibc-multilib on host will fail with extended buildtools. For example,
lib32-luajit fails with extended buildtools.

With a simple search, we have enabled multilib bbclass extension for target gcc [1][2]
{noformat}
$ cat << ENDOF > main.c
#include <stdio.h>
int main() {
  printf("Hello world");
  return 0;
}
ENDOF

$ gcc main.c -o main-64
$ gcc -m32 main.c -o main-32

$ file main-64 main-32
main-64: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=6d51010a38e03f84ea07897cd4ba6fd97ef9c74b, for GNU/Linux 3.2.0, not stripped
main-32: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, BuildID[sha1]=fc8c3058b07d98d2fd19d91a0bf9e35c715c7014, for GNU/Linux 3.2.0, not stripped
{noformat}

[1] https://bugzilla.yoctoproject.org/show_bug.cgi?id=1369
[2] https://git.openembedded.org/openembedded-core/commit/?id=b26819c85881e82ee1b5c68840011e78c321f18e

Add this story is enable multilib for nativesdk gcc, similar like:
'''
root@qemux86-64:~# ./x86_64-buildtools-extended-nativesdk-standalone-5.1.sh -d sdk
root@qemux86-64:~# source sdk/environment-setup-x86_64-pokysdk-linux
root@qemux86-64:~# which gcc
/home/root/sdk/sysroots/x86_64-sdk-linux/usr/bin/gcc
root@qemux86-64:~# gcc main.c -o main-64                             
root@qemux86-64:~# gcc -m32 main.c -o main-32                        
root@qemux86-64:~# file main-64 main-32       
main-64: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter //home/root/sdk/sysroots/x86_64-sdk-linux/lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=3c912c568c7fce674deb3de2978abad0cfe41743, not stripped
main-32: ELF 32-bit LSB pie executable, Intel i386, version 1 (SYSV), dynamically linked, interpreter //home/root/sdk/sysroots/x86_64-sdk-linux/lib/ld-linux.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=49394a5afa840379278cf9e30ed6c7f8a251e850, not stripped
'''

Yocto rejected to add multilib bbclass extension support for nativesdk recipe,
but figure out the root cause:
'''
  the issue is that buildtools-extended-tarball
  doesn't contain 32 bit libgcc/glibc on 64 bit platforms and therefore
  can't build 32 bit binaries.
'''
With a simple search, we could refer the multiconfig selftest case [3],
add a nativesdk recipe nativesdk-multiconfig-multlib-toolchain-packager to
fetch 32 bit libgcc/glibc from 32bit multiconfig build, package it and
install to 64 bit buildtools-extended-tarball. Also reverse to fetch
64 bit libgcc/glibc from 64bit multiconfig build and instll to 
32 bit buildtools-extended-tarball.

[3] https://github.com/openembedded/openembedded-core/blob/master/meta-selftest/recipes-test/multiconfig/multiconfig-image-packager_0.1.bb

Due to the supported SDKMACHINE is:
'''
  aarch64, i586, i686, loongarch64, ppc64, ppc64le, riscv64, x86_64
'''
Only i586 and x86_64, i686 and x86_64 have multilib relationship,
I will choose i686 and x86_64 as the example, mainly includes

- Enhance GCC to enable multilib for i686/x86_64/i586 nativesdk gcc
- Enhance SDK to support multilib dynamic loaders relocation
- Add a nativesdk recipe fetch and install missing libgcc/glibc
- Add a template multlib-nativesdk-gcc to enable the feature for buildtools-extended-tarball build