argdist: STRCMP helper function

argdist: STRCMP helper function

argdist filter expressions can now use the STRCMP helper
function to compare strings. The first string must be a
compile-time constant literal string, and the second string
can be determined at runtime. This is a workaround until
BPF introduces a kernel builtin for strcmp.

Example:

```
argdist -H 'r:c:open(char *file):u64:$latency:STRCMP("test.txt",file)'
```