Make support for thread-unsafe C functions optional.

One of the aspects of CloudABI is that it aims to help you write code
that is thread-safe out of the box. This is very important if you want
to write libraries that are easy to reuse. For CloudABI we decided to
not provide the thread-unsafe functions. So far this is working out
pretty well, as thread-unsafety issues are detected really early on.

The following patch adds a knob to libc++,
_LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS, that can be set to disable
thread-unsafe functions that can easily be avoided in practice. The
following functions are not thread-safe:

- <clocale>: locale handles should be preferred over setlocale().
- <cstdlib>: mbrlen(), mbrtowc() and wcrtomb() should be preferred over
  their non-restartable counterparts.
- <ctime>: asctime(), ctime(), gmtime() and localtime() are not
  thread-safe. The first two are also deprecated by POSIX.

Differential Revision:	http://reviews.llvm.org/D8703
Reviewed by:	marshall


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@240527 91177308-0d34-0410-b5e6-96231b3b80d8

include/cstdlib

@@ -147,9 +147,11 @@ using ::ldiv;
 #undef lldiv
 using ::lldiv;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 using ::mblen;
 using ::mbtowc;
 using ::wctomb;
+#endif
 using ::mbstowcs;
 using ::wcstombs;
 #ifdef _LIBCPP_HAS_QUICK_EXIT