Note: This is not needed anymore as there is a way to patch Widevine cdm.
So no need to patch glibc 2.41 any more...
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Before we start to patch and compile glibc we will have to download and install Stuart Winter´s slackkit-*-aarch64-1.txz.
From here: https://slackware.uk/slackwarearm/slackwareaarch64-current/slackware/d/

And we also have to download "slackware64-current/source/l/glibc" and "slackwarearm/slackwareaarch64-current/source/l/glibc/" with rsync

As root
   # cd /usr/src
   # wget https://slackware.uk/slackwarearm/slackwareaarch64-current/slackware/d/slackkit-1.47-aarch64-1.txz
   # installpkg slackkit-1.47-aarch64-1.txz

   # cd /root
   # mkdir -p slackware64-current/source/l/glibc/
   # cd slackware64-current/source/l/glibc/

As we only need and want glibc directory
Sync the Slackware/x86_64 tree for glibc:
   # rsync -Pavv ftp.slackware.org.uk::slackware/slackware64-current/source/l/glibc/ .
   # cd
   # mkdir -p slackwarearm/slackwareaarch64-current/source/l/glibc
   # cd slackwarearm/slackwareaarch64-current/source/l/glibc

We also need glibc directory from aarch64 source
   # rsync -Pavv ftp.slackware.org.uk::slackwarearm/slackwareaarch64-current/source/l/glibc/ .
   # mv glibc.SlackBuild glibc.SlackBuild.aarch64
   # wget https://raw.githubusercontent.com/mostman/Slackbuilds/Slackwareaarch64-current/glibc/glibc.SlackBuild
   # wget https://github.com/mostman/Slackbuilds/raw/Slackwareaarch64-current/glibc/glibc-do_not_check-GLIBC_ABI_DT_RELR-chrome-tls-libwidevine.patch.gz
   # cp glibc-do_not_check-GLIBC_ABI_DT_RELR-chrome-tls-libwidevine.patch.gz /root/slackware64-current/source/l/glibc/

Set correct permissions
   # chmod 755 glibc.SlackBuild
   # cd arm
   # mv build build.aarch64
   # wget https://raw.githubusercontent.com/mostman/Slackbuilds/Slackwareaarch64-current/glibc/arm/build
   # chmod 755 build
   # cd ..

And now we can compile glibc
   # ./arm/build
 
Packages will be stored under "/root/tgzstash"
   # cd /root/tgzstash
   # upgradepkg a/aaa_glibc-solibs-2.41-aarch64-1p1.txz l/glibc-2.41-aarch64-1p1.txz
 
 And you are all done.