UserspaceEmulator+LibX86: Turn on -O3 optimization for emulation code

Since this code is performance-sensitive, let's have the compiler do
whatever it can to help us with the most important files.

This yields a ~8% speedup.

DevTools/UserspaceEmulator/SoftCPU.cpp

@@ -30,6 +30,10 @@
 #include <stdio.h>
 #include <string.h>
 
+#if defined(__GNUC__) && !defined(__clang__)
+#    pragma GCC optimize("O3")
+#endif
+
 //#define MEMORY_DEBUG
 
 namespace UserspaceEmulator {