[Issue 24593] New: Some structs in core.sys.posix.sys.types should be union instead of struct

[d-bugmail at puremagic.com]

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

          Issue ID: 24593
           Summary: Some structs in core.sys.posix.sys.types should be
                    union instead of struct
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: tim.dlang at t-online.de

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

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