runtime/cgo: remove memset in _cgo_sys_thread_start on linux
pthread_attr_init in glibc and musl libc already explicitly clear the
pthread_attr argument before setting it, see
https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_attr_init.c
and https://git.musl-libc.org/cgit/musl/log/src/thread/pthread_attr_init.c
It looks like pthread_attr_init has been implemented like this for a
long time in both libcs. The comment and memset in _cgo_sys_thread_start
probably stem from a time where not all libcs did the explicit memset in
pthread_attr_init.
Also, the memset in _cgo_sys_thread_start is not performed on all linux
platforms further indicating that this isn't an issue anymore.
Change-Id: I920148b5d24751195ced7af5bb7c52a7f8293259
Reviewed-on: https://go-review.googlesource.com/c/go/+/404275
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>