Make system_error::message() thread safe. Fixes PR25598.

Summary:
system_error::message() uses `strerror` for the generic and system categories. This function is not thread safe.

The fix is to use `strerror_r`. It has been available since 2001 for GNU libc and since BSD 4.4 on FreeBSD/OS X.
On platforms with GNU libc the extended version is used which always returns a valid string, even if an error occurs.

In single-threaded builds `strerror` is still used.

See https://llvm.org/bugs/show_bug.cgi?id=25598

Reviewers: majnemer, mclow.lists

Subscribers: erik65536, cfe-commits, emaste

Differential Revision: http://reviews.llvm.org/D20903

llvm-svn: 272633