Fix printf$LDBLStub workaround for Darwin

Apparently, the original fix never really worked due to typos and oversights.

configure.ac

@@ -1175,8 +1175,8 @@ AC_CHECK_LIB(rt, clock_gettime)
 AC_CHECK_FUNCS(clock_gettime)
 
 dnl ** check for Apple's "interesting" long double compatibility scheme
-AC_MSG_CHECKING(for printf$LDBLStub)
-AC_TRY_LINK_FUNC(printf$LSBLStub,
+AC_MSG_CHECKING(for printf\$LDBLStub)
+AC_TRY_LINK_FUNC(printf\$LDBLStub,
     [
         AC_MSG_RESULT(yes)
         AC_DEFINE([HAVE_PRINTF_LDBLSTUB],[1],

rts/Linker.c

@@ -797,6 +797,7 @@ static RtsSymbolVal rtsSyms[] = {
       RTS_POSIX_ONLY_SYMBOLS
       RTS_MINGW_ONLY_SYMBOLS
       RTS_CYGWIN_ONLY_SYMBOLS
+      RTS_DARWIN_ONLY_SYMBOLS
       RTS_LIBGCC_SYMBOLS
 #if defined(darwin_HOST_OS) && defined(i386_HOST_ARCH)
       // dyld stub code contains references to this,