trace: Allow function signatures in uprobes and kprobes

trace: Allow function signatures in uprobes and kprobes

§`trace` now allows uprobes and kprobes to have function signatures,
which means function parameters can be named and typed, rather than
relying on the positional arg1, arg2, etc. arguments. This also
enables structure field access, which is impossible with the unnamed
arguments due to rewriter limitations.

The example requested by @brendangregg, which now works, is the
following:

§Â```
PID    TID    COMM         FUNC             -
777    785    automount    SyS_nanosleep    sleep for 500000000 ns
777    785    automount    SyS_nanosleep    sleep for 500000000 ns
777    785    automount    SyS_nanosleep    sleep for 500000000 ns
777    785    automount    SyS_nanosleep    sleep for 500000000 ns
^C
```