hide all libv8 symbols on ELF targets

This gives a nice binary size reduction and may allow us to eliminate
the mini_racer_loader shim:

         text           data    bss
before:  63987419       718108  110676
 after:  56535503       454196  110676

append_ldflags is available in Ruby 2.3+ mkmf.rb, allowing us
to probe for supported LDFLAGS without extra conditionals.

ext/mini_racer_extension/extconf.rb

@@ -70,6 +70,9 @@
 
 Libv8::Node.configure_makefile
 
+# --exclude-libs is only for i386 PE and ELF targeted ports
+append_ldflags("-Wl,--exclude-libs=ALL ")
+
 if enable_config('asan')
   $CXXFLAGS.insert(0, " -fsanitize=address ")
   $LDFLAGS.insert(0, " -fsanitize=address ")