libc: export pthread_getname_np stub

pthread_getname_np needs to be provided by libc in order to import
jemalloc 5.3.0.

A stub implementation for libc pthread_getname_np() is added for
_pthread_stubs.c, which always reports empty name for the main thread.

Internal _pthread_getname_np() is not exported, but provided for libc
own use.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41461