Removing API usage not applicable on iOS (stat and fstat) in libhermes

Summary:
This diff just removes references to functions that use (either
directly or transitively) those functions, and relies on linker DCE
to clean up the rest. Trying to extricate all uses of these functions
would otherwise result in very significant changes.

This diff makes two changes:
1. In raw_fd_ostream, fall back to a default buffer size when building
for iOS.
2. Update a function in LLVM that was marked as "used" to suppress
warnings, to instead use the new `[[maybe_unused]]` attribute. This
allows the function, and the calls to stat it retains, to be stripped.

Reviewed By: tmikov, TheSavior

Differential Revision: D53133166

fbshipit-source-id: f425762bc6b7072cbd8235c65e9cf652cecb981c

external/llvh/lib/Support/Signals.cpp

@@ -103,7 +103,7 @@ static FormattedNumber format_ptr(void *PC) {
 }
 
 /// Helper that launches llvm-symbolizer and symbolizes a backtrace.
-LLVM_ATTRIBUTE_USED
+[[maybe_unused]]
 static bool printSymbolizedStackTrace(StringRef Argv0, void **StackTrace,
                                       int Depth, llvh::raw_ostream &OS) {
   if (DisableSymbolicationFlag)

external/llvh/lib/Support/raw_ostream.cpp

@@ -764,7 +764,7 @@ size_t raw_fd_ostream::preferred_buffer_size() const {
   if (IsWindowsConsole)
     return 0;
   return raw_ostream::preferred_buffer_size();
-#elif !defined(__minix)
+#elif !defined(__minix) && !(defined(__APPLE__) && defined(HERMES_IS_MOBILE_BUILD))
   // Minix has no st_blksize.
   assert(FD >= 0 && "File not yet open!");
   struct stat statbuf;