kmsan: Use __builtin_memset to initialize per-thread state

Accesses to KMSAN's TLS block are not instrumented, so there's no need
to use kmsan_memset(). No functional change intended.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.

(cherry picked from commit e5caed14067b40f1454d74e99789a28508d0eea3)