tests: extend test coverage of mmap syscall

Check decoding of "old mmap" edition of mmap syscall
on those architectures that define it.

* tests/old_mmap.c: New file.
* tests/old_mmap.test: New test.
* tests/.gitignore: Add old_mmap.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add old_mmap.test.
diff --git a/tests/.gitignore b/tests/.gitignore
index f7fbbb3..5b3cb43 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -120,6 +120,7 @@
 netlink_inet_diag
 netlink_unix_diag
 newfstatat
+old_mmap
 oldselect
 open
 openat