Support OpenBSD in common interceptors

Summary:
Extract  INIT_PTHREAD_ATTR_GET_SCHED from INIT_PTHREAD_ATTR_GET,
as the former is not supported on OpenBSD.

Supported interceptors
getdetachstate, getguardsize, getscope, getstacksize, getgrouplist and getstack
Unsupported
getschedparam, getgroupmembership and getschedpolicy

Patch by David CARLIER

Reviewers: krytarowski, vitalybuka

Reviewed By: vitalybuka

Subscribers: fedor.sergeev, srhines, kubamracek, mgorny, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D44035

llvm-svn: 326653