runtime: fix compile failure from use of _Unwind_Backtrace() on FreeBSD

FreeBSD uses libunwind from <https://github.com/libunwind/libunwind>.  Its
<unwind.h> guards _Unwind_Backtrace with _GNU_SOURCE guards.  Define _GNU_SOURCE
so that withCurrentBacktraceImpl() can access _Unwind_Backtrace.

Note that there are quite a few <unwind.h>s:

1. gcc provides one as part of its libgcc (FreeBSD does not use gcc and provides
   its own libgcc that does not have an <unwind.h>)
2. LLVM's libunwind provides one (FreeBSD does not use LLVM's libcxxabi or
   LLVM's libunwind)
3. <https://github.com/libunwind/libunwind> provides one (FreeBSD has this one)
4. clang provides one (FreeBSD *has* this one, but the header search path for
   C++ prefers swiftlang#3)

Author: mhjacobson

stdlib/public/runtime/Errors.cpp

@@ -22,6 +22,25 @@
 #include <mutex>
 #endif
 
+#if defined(__ELF__)
+# if defined(__FreeBSD__) && !defined(_GNU_SOURCE)
+// In order to access _Unwind_Backtrace() from (the non-LLVM, non-GCC)
+// <unwind.h> in FreeBSD, define _GNU_SOURCE around the include.  This is
+// similar to what clang's unwind.h does.
+// Do this before including other headers which may in turn include <unwind.h>,
+// which has a header guard.
+#  define _GNU_SOURCE
+#  define _SHOULD_UNDEFINE_GNU_SOURCE
+# endif
+
+#include <unwind.h>
+
+# ifdef _SHOULD_UNDEFINE_GNU_SOURCE
+#  undef _GNU_SOURCE
+#  undef _SHOULD_UNDEFINE_GNU_SOURCE
+# endif
+#endif
+
 #include <inttypes.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -56,10 +75,6 @@
 #include <android/log.h>
 #endif
 
-#if defined(__ELF__)
-#include <unwind.h>
-#endif
-
 #include <inttypes.h>
 
 #ifdef SWIFT_HAVE_CRASHREPORTERCLIENT