Sync (make same) the offsetof macro definition in include/ with the
definition of the same in sys/sys/. The problem was discovered while
working on implementing a new C11 gets_s() for libc. (The new gets_s()
requires rsize_t found in include/stddef.h.) The solution to sync the two
definitions was suggested by ed@ while discussing D12667.

Suggested by: ed

MFC 309274:
Use the correct name for the GCC macro indicating max_align_t is defined.

MFC 307756: Define max_align_t for C11.

libc++'s stddef.h includes an existing definition of max_align_t for
C++11, but it is only defined for C++, not for C. In addition, GCC and
clang both define an alternate version of max_align_t that uses a
union of multiple types rather than a plain long double as in libc++.
This adds a __max_align_t to <sys/_types.h> that matches the GCC and
clang definition that is mapped to max_align_t in <stddef.h>.

PR: 210890

MFC r263998:

Rename __wchar_t so it no longer conflicts with __wchar_t from clang 3.4
-fms-extensions.

MFC r263998:

Rename __wchar_t so it no longer conflicts with __wchar_t from clang 3.4
-fms-extensions.