aarch64: Clear TLS registers during exec().

These are not stored in the trapframe so must be cleared explicitly.

This is similar to one of the MIPS changes in 822d2d6ac94f.

Reviewed by: andrew
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D28711