[libcxxabi] Disable calls to fprintf for baremetal targets.

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

Differential Revision: https://reviews.llvm.org/D30339

llvm-svn: 296136

libcxxabi/src/abort_message.cpp

@@ -35,6 +35,7 @@ __attribute__((visibility("hidden"), noreturn))
 void abort_message(const char* format, ...)
 {
     // write message to stderr
+#if !defined(NDEBUG) && !defined(LIBCXXABI_BAREMETAL)
 #ifdef __APPLE__
     fprintf(stderr, "libc++abi.dylib: ");
 #endif
@@ -43,6 +44,7 @@ void abort_message(const char* format, ...)
     vfprintf(stderr, format, list);
     va_end(list);
     fprintf(stderr, "\n");
+#endif
 
 #if defined(__APPLE__) && defined(HAVE_CRASHREPORTERCLIENT_H)
     // record message in crash report