sysent: add a new NORETURN type flag

System calls of type NORETURN don't return and their stubs are declare
not to.

Reviewed by:	kevans, kib
Differential Revision:	https://reviews.freebsd.org/D51673

sys/kern/syscalls.master

@@ -51,6 +51,7 @@
 ;	SYSMUX	syscall multiplexer.  No prototype, argument struct, or
 ;		handler is declared or used.  Handled in MD syscall code.
 ;	CAPENABLED syscall is allowed in capability mode
+;	NORETURN the syscall does not return
 ;
 ; To support programmatic generation of both the default ABI and 32-bit compat
 ; (freebsd32) we impose a number of restrictions on the types of system calls.

sys/tools/syscalls/core/syscall.lua

@@ -28,6 +28,7 @@ syscall.known_flags = util.set {
 	-- flags beyond this point are modifiers
 	"CAPENABLED",
 	"NOLIB",
+	"NORETURN",
 	"NOTSTATIC",
 	"SYSMUX",
 }

sys/tools/syscalls/scripts/libsys_h.lua

@@ -76,8 +76,12 @@ function libsys_h.generate(tbl, config, fh)
 
 	for _, v in pairs(s) do
 		if print_decl(v) then
-			gen:write(string.format("%s __sys_%s(%s);\n",
-			    v.ret, v.name, v.argstr_type_var))
+			local ret_attr = "";
+			if v.type.NORETURN then
+				ret_attr = "_Noreturn "
+			end
+			gen:write(string.format("%s%s __sys_%s(%s);\n",
+			    ret_attr, v.ret, v.name, v.argstr_type_var))
 		end
 	end