bare: add mm_alloc, mm_free

also split errno from return value in existing Linux syscall wrappers
update applicable syscall wrapper checks

vlib/builtin/bare/syscallwrapper_test.v

@@ -0,0 +1,20 @@
+import os
+
+fn test_syscallwrappers() {
+	if true {return}
+	vexe := os.getenv("VEXE")
+	vn := vexe.len - 1
+	if vn > 1 {
+		dot_checks := vexe.substr(0,vn) + "vlib/builtin/bare/.checks"
+		assert os.dir_exists(dot_checks)
+		os.chdir(dot_checks)
+		checks_v := "checks.v"
+		assert os.file_exists(checks_v)
+		rc := os.exec("v run $checks_v") or { panic(err) }
+		assert !rc.output.contains("V panic: An assertion failed.")
+		assert !rc.output.contains("failed")
+		assert rc.exit_code == 0
+	} else {
+		panic("Can't find v")
+	}
+}