makesyscalls.lua: trim trailing spaces/commas from args

These are insignificant as far as declarations go, and we've historically
allowed it. fhlinkat in ^/sys/kern/syscalls.master, for example, currently
has a trailing comma after its final argument that this version of
makesyscalls is ignoring (not by conscious decision).

Fix it for now by actively stripping off trailing whitespace/commas until
we decide to actively prohibit it.

sys/tools/makesyscalls.lua

@@ -1010,6 +1010,7 @@ process_syscall_def = function(line)
 			abort(1, "Not a signature? " .. line)
 		end
 		args = line:match("^[^(]+%((.+)%)[^)]*$")
+		args = trim(args, '[,%s]')
 	end
 
 	::skipalt::