- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Import new libcxxrt / libc++. This brings some bug fixes, including a potential race condition for static initialisers.

Merge libcxxrt c812a07cd2f95c1403baf0bbe0366e7618d1d6d3:

* Don't call the _fast version of the TLS accessor in terminate() or
unexpected().
1) TLS may not have been set up yet.
2) When we're in one of these functions, Really Bad Stuff has
happened and potentially saving a few cycles really isn't
important.
* Merge in fixes from FreeBSD trunk to make atomics work with recent
clang.

MFC after: 1 week

Fix libcxxrt / libc++ build with the clang in head.

Pointy hat to: theraven

Merge new version of libcxxrt. This brings in three fixes:

- Don't treat pointers to members as pointers in catch blocks (they're usually
fat pointers).

- Correctly catch foreign exceptions in catchalls.

- Ensure that a happens-before relationship is established when setting
terminate handlers in one thread and calling them in another.

Import b9db3a010143160624f123763025ab544b69bd9a of libcxxrt. This brings in
three fixes:

- Don't treat pointers to members as pointers in catch blocks (they're usually
fat pointers).

- Correctly catch foreign exceptions in catchalls.

- Ensure that a happens-before relationship is established when setting
terminate handlers in one thread and calling them in another.