Tests: Fix: arm64 use sys_openat instead of sys_open

gen-syscall-events is failing to build on arm64 because of the following
error:
```
gen-syscall-events.c: In function ‘main’:
gen-syscall-events.c:35:15: error: ‘SYS_open’ undeclared (first use in
this function)
  fd = syscall(SYS_open, "/proc/cpuinfo", O_RDONLY);
```

SYS_open is not available using the syscall(2) arm64 glibc function.
SYS_openat should be used instead.

Change test app and test cases to use SYS_openat.

Other projects have encountered the same issue:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=758521
https://bugs.launchpad.net/linaro-aarch64/+bug/1100782

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>