Some structs in core.sys.posix.sys.types should be union instead of struct

[dlangBugzillaToGithub]

Tim reported this on 2024-06-09T14:23:18Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=24593

Description

The types pthread_rwlock_t, pthread_rwlockattr_t, pthread_barrier_t, pthread_barrierattr_t are unions in the glibc headers, but structs in druntime. As a result they are bigger in druntime.

Found by comparing type sizes in druntime with type sizes found with ImportC: https://github.com/dlang/dmd/pull/16571