benchmark: Add new syscall benchmark suite

Add some simple benchmarks to test performance of syscall entry and exit
logic. For now, we have 3 benchmarks, getpid, getppid and invalid. The
first 2 call the respective syscalls, the 3rd one makes a syscall with
an invalid syscall number.

Benchmark yamls are added to the benchmark library:

  syscall/getpid.yaml
  syscall/getppid.yaml
  syscall/invalid.yaml

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>