`__data_loc` fields are dynamically sized by the kernel at
runtime. The field data follows the tracepoint structure entry,
and needs to be extracted in a special way. The `__data_loc` field
itself is a 32-bit value that consists of two 16-bit parts: the
high 16 bits are the length of the data, and the low 16 bits are
the offset of the data from the beginning of the tracepoint
structure. From a cursory look, there are >200 tracepoints in
recent kernels that have this kind of field.

This patch fixes `tp_frontend_action.cc` to recognize and emit
`__data_loc` fields correctly, as 32-bit opaque fields. Then, it
introduces two helper macros:

`TP_DATA_LOC_READ(dst, field)` reads from `args->field` by finding
the right offset and length and emitting the `bpf_probe_read`
required to fetch the data. This will only work with new kernels.

`TP_DATA_LOC_READ_CONST(dst, field, length)` takes a user-specified
length rather than finding it from `args->field`. This will work
on older kernels, where the BPF verifier doesn't allow non-constant
sizes to be passed to `bpf_probe_read`.