[libunwind] Fflush stderr after each log message

[mstorsjo]

Summary

In most configs, stderr is line buffered by default, but in some
cases on Windows (running in git bash, or running in Wine) stderr
can end up fully buffered.

See 2ec75a0869ab01fa9caf310e8a31eb7716182d30 for a similar change
for the output from lit itself.

This has no effect on libunwind when the log messages aren't enabled
via the environment variables.

[compnerd]

If you're enabling libunwinding logging, you are already in a bad place. The additional flush isn't going to hurt you any further.

[mstorsjo]

In D147632#4247202, @compnerd wrote:

If you're enabling libunwinding logging, you are already in a bad place. The additional flush isn't going to hurt you any further.

Yep, and stderr is normally line buffered anyway so there shouldn't really be any difference for other users anyway.

libunwind/src/config.h

do {                                                                         \
     abort();                                                                   \
   } while (0)
 #endif
 
 #if defined(NDEBUG) && defined(_LIBUNWIND_IS_BAREMETAL)
 #define _LIBUNWIND_LOG0(msg)
 #define _LIBUNWIND_LOG(msg, ...)
 #else
-#define _LIBUNWIND_LOG0(msg)                                               \
-  fprintf(stderr, "libunwind: " msg "\n")
-#define _LIBUNWIND_LOG(msg, ...)                                               \
-  fprintf(stderr, "libunwind: " msg "\n", __VA_ARGS__)
+#define _LIBUNWIND_LOG0(msg) do {                                              \
+    fprintf(stderr, "libunwind: " msg "\n");                                   \
+    fflush(stderr);                                                            \
+  } while (0)
+#define _LIBUNWIND_LOG(msg, ...) do {                                          \
+    fprintf(stderr, "libunwind: " msg "\n", __VA_ARGS__);                      \
+    fflush(stderr);                                                            \
+  } while (0)
 #endif
 
 #if defined(NDEBUG)
   #define _LIBUNWIND_LOG_IF_FALSE(x) x
 #else
   #define _LIBUNWIND_LOG_IF_FALSE(x)                                           \
     do {                                                                       \
       bool _ret = x;                                                           \