[libunwind] Disable calls to fprintf when building for baremetal targets in release mode

[rs]

Summary

We've been having issues with using libcxxabi and libunwind for baremetal targets because fprintf is dependent on io functions, this patch disable calls to fprintf when building for baremetal targets in release mode.

libcxxabi patch is here https://reviews.llvm.org/D30339

[jroelofs]

LGTM

src/config.h

#if defined(__i386__) || defined(__x86_64__) ||                                \
     (!defined(__APPLE__) && defined(__arm__)) ||                               \
     (defined(__arm64__) || defined(__aarch64__)) ||                            \
     (defined(__APPLE__) && defined(__mips__))
 #define _LIBUNWIND_BUILD_ZERO_COST_APIS 1
 #else
 #define _LIBUNWIND_BUILD_ZERO_COST_APIS 0
 #endif
 
+#if defined(NDEBUG) && defined(_LIBUNWIND_IS_BAREMETAL)
+#define _LIBUNWIND_ABORT(msg)                                                  \
+  do {                                                                         \
+    abort();                                                                   \
+  } while (0)
+#else
 #define _LIBUNWIND_ABORT(msg)                                                  \
   do {                                                                         \
     fprintf(stderr, "libunwind: %s %s:%d - %s\n", __func__, __FILE__,          \
             __LINE__, msg);                                                    \
     fflush(stderr);                                                            \
     abort();                                                                   \
   } while (0)
+#endif
 
+#if defined(NDEBUG) && defined(_LIBUNWIND_IS_BAREMETAL)
+#define _LIBUNWIND_LOG(msg, ...)
+#else
 #define _LIBUNWIND_LOG(msg, ...)                                               \
   fprintf(stderr, "libunwind: " msg "\n", __VA_ARGS__)
+#endif
 
 #if defined(_LIBUNWIND_HAS_NO_THREADS)
   // only used with pthread calls, not needed for the single-threaded builds
   #define _LIBUNWIND_LOG_NON_ZERO(x)
 #else
   #if defined(NDEBUG)
     #define _LIBUNWIND_LOG_NON_ZERO(x) x
   #else