Download or recompile glibc from the slackbuild scripts found here.
If you dont want to hazzle with compiling it, i have added precompiled and patched (Added Widevine support for TLS with 64-byte) binaries here.

If you decide to compile glibc your self, i recommend you to read and install Stuart Winter´s slackkit and documentation for Slackware Aarch64 as its his glibc slackbuild that i used to add the patches to and compile glibc natively.

If you install the precompiled binaries make sure you use the command "upgradepkg aaa_glibc-solibs-2.37-aarch64-1p1.txz glibc-2.37-aarch64-1p1.txz"
and it has to be in this order or you will have problems...

Note: Both aaa_glibc-solibs and glibc packages are needed or it will not work to load libwidevine...

Enjoy

Best regards
Minime