Native access to Genode environment

Add a 'GenodeEnv' type and a 'componentConstructHook' to the system
module. The 'componentConstructHook' allows for detection of POSIX style
programs that exit implicitly or native Genode components that
initialize to serve RPC requests and OS signals.

This hook takes a 'GenodeEnv' argument so that the environment interface
is passed cleanly to application code after globals are initialized.
This is an typed pointer to a C++ object, procedures for accessing the
environment will be available from a Nimble library and not included in
the standard library.

The standard library has an internal pointer to the environment object
but this is not for external use, the undocumented global environment
pointer has been removed.

compiler/cgen.nim

@@ -1035,14 +1035,19 @@ proc genMainProc(m: BModule) =
       "}$N$N"
 
     GenodeNimMain =
-      "Libc::Env *genodeEnv;$N" &
+      "extern Genode::Env *nim_runtime_env;$N" &
+      "extern void nim_component_construct(Genode::Env*);$N$N" &
       NimMainBody
 
     ComponentConstruct =
       "void Libc::Component::construct(Libc::Env &env) {$N" &
-      "\tgenodeEnv = &env;$N" &
+      "\t// Set Env used during runtime initialization$N" &
+      "\tnim_runtime_env = &env;$N" &
       "\tLibc::with_libc([&] () {$N\t" &
+      "\t// Initialize runtime and globals$N" &
       MainProcs &
+      "\t// Call application construct$N" &
+      "\t\tnim_component_construct(&env);$N" &
       "\t});$N" &
       "}$N$N"
 
@@ -1059,6 +1064,7 @@ proc genMainProc(m: BModule) =
   elif platform.targetOS == osGenode:
     nimMain = GenodeNimMain
     otherMain = ComponentConstruct
+    m.includeHeader("<libc/component.h>")
   elif optGenDynLib in m.config.globalOptions:
     nimMain = PosixNimDllMain
     otherMain = PosixCDllMain