Revert r330897:

This was intended to be a non-functional change. It wasn't. The commit
message was thus wrong. In addition it broke arm, and merged crypto
related code.

Revert with prejudice.

This revert skips files touched in r316370 since that commit was since
MFCed. This revert also skips files that require $FreeBSD$ property
changes.

Thank you to those who helped me get out of this mess including but not
limited to gonzo, kevans, rgrimes.

Requested by: gjb (re)

Partial merge of the SPDX changes

These changes are incomplete but are making it difficult
to determine what other changes can/should be merged.

No objections from: pfg

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Simply disallow <stdnoreturn.h> to be used in combination with C++.

There is no way one could possibly use this header file in combination
with C++ code. The problem is that in C11 the `noreturn' macro expands
to the `_Noreturn' function specifier, while in C++11 the `noreturn'
keyword is an attribute.

So in C11 you have to write:

noreturn void exit(int status);

While in C++11 you have to write:

[[noreturn]] void exit(int status);

It is impossible to #define noreturn for C++ in such a way that it
allows both conventions.

By intentionally breaking this header this way, we prevent people from
using this header in their C++<11 sources.

Add <stdalign.h> and <stdnoreturn.h>.

Even though these header files make little sense to me, they are part of
the standard. By including these header files, you can simply use
`alignas', `alignof' and `noreturn' instead of the underscore-prefixed
versions.