Add a runtime switch to turn off LibXSMM

[jermainewang]

🚀 Feature

Add an API dgl.use_libxsmm(flag). If the flag is True, turn on libxsmm. Otherwise, disable it.

Motivation

To resolve the issue that LibXSMM is not supported by some platform #3459

Alternatives

Let users compile from source without LibXSMM, but it's not a preferred solution for most users.

[jermainewang]

#3471