[PATCH 0/6] MR7250: ntdll: Properly implement NtFlushProcessWriteBuffers (try 2).

[Jinoh Kang (＠iamahuman)]

Supersedes !741.

On macOS 10.14+ `thread_get_register_pointer_values` is called on every thread of the process.  
On Linux 4.14+ `membarrier(MEMBARRIER_CMD_GLOBAL_EXPEDITED, ...)` is used.  
On x86 Linux <= 4.13 and on other platforms it falls back to calling `NtGetContextThread()` on each thread.

The fast path patches from @tmatthies are slightly modified in the following ways:

1. On unsupported platforms, the `try_*()` function returns `FALSE` instead of `0`.
2. `try_exp_membarrier()` is called first, then `try_mach_tgrpvs()`.