syscall_emul: [patch 2/22] move SyscallDesc into its own .hh and .cc

The class was crammed into syscall_emul.hh which has tons of forward
declarations and template definitions. To clean it up a bit, moved the
class into separate files and commented the class with doxygen style
comments. Also, provided some encapsulation by adding some accessors and
a mutator.

The syscallreturn.hh file was renamed syscall_return.hh to make it consistent
with other similarly named files in the src/sim directory.

The DPRINTF_SYSCALL macro was moved into its own header file with the
include the Base and Verbose flags as well.

--HG--
rename : src/sim/syscallreturn.hh => src/sim/syscall_return.hh

src/sim/SConscript

@@ -80,6 +80,7 @@ if env['TARGET_ISA'] != 'null':
     Source('fd_entry.cc')
     Source('pseudo_inst.cc')
     Source('syscall_emul.cc')
+    Source('syscall_desc.cc')
 
 DebugFlag('Checkpoint')
 DebugFlag('Config')