style: [patch 3/22] reduce include dependencies in some headers

Used cppclean to help identify useless includes and removed them. This
involved erroneously included headers, but also cases where forward
declarations could have been used rather than a full include.

src/arch/alpha/process.cc

@@ -40,6 +40,7 @@
 #include "mem/page_table.hh"
 #include "sim/byteswap.hh"
 #include "sim/process_impl.hh"
+#include "sim/syscall_return.hh"
 #include "sim/system.hh"
 
 using namespace AlphaISA;

src/arch/alpha/process.hh

@@ -32,6 +32,7 @@
 #ifndef __ARCH_ALPHA_PROCESS_HH__
 #define __ARCH_ALPHA_PROCESS_HH__
 
+#include "mem/page_table.hh"
 #include "sim/process.hh"
 
 class AlphaLiveProcess : public LiveProcess