[PATCH] Linux: Implement membarrier function

[Florian Weimer]

* Adhemerval Zanella:

> If we are replicating the values, meaning the idea is to keep it sync at least
> when we have the minimum supported kernel of 4.16, why just not add a comment
> to add linux/membarrier.h once the minimum supported kernel provides this
> header and not rely on linux/membarrier.h?

This way, we can compile the test with any supported kernel headers for
glibc.

If we defer to <linux/membarrier.h> unconditionally, we cannot build the
test with all kernel headers.  The current approach definitely makes the
test case much cleaner.

> Also I think the minimum kernel that provides this header is 4.3, however
> by using 4.3 as the condition to include the kernel header in add another
> issue which is glibc will have different semantic depending of the installed
> header. This fallback enum definition is also lacking MEMBARRIER_CMD_SHARED,
> and although is provided by kernel headers just for compatibility, it is 
> another interface difference it has depending of the installed kernel
> header.

MEMBARRIER_CMD_SHARED is included.

Thanks,
Florian