Synchronize libcxxrt in stable/9 with stable/{10,11} and head.

MFC r284549 (by emaste):

Import libcxxrt master e64e93fe5bba67a6d52cbe5a97f8770c054bfa65

This includes a number of fixes to the C++ demangler (obtained from
upstream ELF Tool Chain).

MFC r284551 (by emaste):

Import libcxxrt master e64e93fe5bba67a6d52cbe5a97f8770c054bfa65.

This includes a number of demangler fixes obtained from upstream
ELF Tool Chain.

PR: 200913
Sponsored by: The FreeBSD Foundation

MFC r284553 (by emaste):

Update libcxxrt upgrade instructions

The typeinfo file no longer exists upstream.

MFC r288830:

Add std::uncaught_exceptions() to libcxxrt (C++17, see N4152 and N4259).
This has also been submitted upstream.

MFC r288830:

Add std::uncaught_exceptions() to libcxxrt (C++17, see N4152 and N4259).
This has also been submitted upstream.

MFC r297299:

Compile libcxxrt as C++11, since it is only really used in combination
with libc++, which is also C++11. Also change one _Static_assert (which
is really C11) back into static_assert, like upstream.

This should help when compiling libcxxrt with newer versions of gcc,
which refuse to recognize any form of static assertions, if not
compiling for C++11 or higher.

While here, add -nostdinc++ to CFLAGS, to prevent picking up any C++
headers outside the source tree.

MFC r299144:

Import libcxxrt master 516a65c109eb0a01e5e95fbef455eb3215135cef.

Interesting fixes:
3adaa2e Fix _Unwind_Exception cleanup functions
286776c Check exception cleanup function ptr before calling
edda626 Correct exception specifications on new and delete operators

MFC r303157 (by emaste):

libcxxrt: add padding in __cxa_allocate_* to fix alignment

The addition of the referenceCount to cxa_allocate_exception put the
unwindHeader at offset 0x58 in cxa_exception, but it requires 16-byte
alignment. In order to avoid changing the current __cxa_exception ABI
(and thus breaking its consumers), add explicit padding in the
allocation routines (and account for it when freeing).

This is intended as a lower-risk change for FreeBSD 11. A "more correct"
fix should be prepared for upstream and -CURRENT.

Reviewed by: dim
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7271

MFC r303400 (by emaste):

libcxxrt: fix demangling of wchar_t

'wchar_t' is 7 characters long, not 6. r303297 fixed this in libelftc,
but not the second copy of this file that we have in libcxxrt.

PR: 208661
Submitted by: Daniel McRobb
Obtained from: ELF Tool Chain r3480

MFC r305396:

Add _US_ACTION_MASK to libcxxrt's arm-specific unwind header. This
value is used in newer versions of compiler-rt.

MFC r315947 | dim | 2017-03-25 14:17:48 +0100 (Sat, 25 Mar 2017) | 8 lines

Import libcxxrt master 8a853717e61d5d55cbdf74d9d0a7545da5d5ff92.

Interesting fixes which were not already merged:
0c7c611 Merge C++ demangler bug fixes from ELF Tool Chain (#40)
2b208d9 __cxa_demangle_gnu3: demangle 'z' as '...', not 'ellipsis' (#41)