runtime/cgo: remove memset in _cgo_sys_thread_start on freebsd/arm

runtime/cgo: remove memset in _cgo_sys_thread_start on freebsd/arm

pthread_attr_init on freebsd properly initializes the pthread_attr,
there is no need to zero it before the call. The comment and code were
probably copied from the linux/arm implementation.

This aligns the implementation on freebsd/arm with the implementation on
other freebsd architectures.

Fixes #44248

Change-Id: If82ebb115b877b6c6f4862018a9419ba8d870f12
Reviewed-on: https://go-review.googlesource.com/c/go/+/393617
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Hajime Hoshi <hajimehoshi@gmail.com>
Trust: Hajime Hoshi <hajimehoshi@gmail.com>