Remote query for structure layout

Hi everybody,

I would like to propose a new command for the remote protocol that can be used to
query structure layouts.
Essentially a `qSymbol` equivalent for the output of `ptype`.

Background:

The RTOS-support in OpenOCD has to read in-memory datastructures of the debug
target to reconstruct the threading information for GDB.
For example for FreeRTOS this is done by first looking up the scheduler
datastructure via `qSymbol` and then doing hardcoded pointer arithmetic based on
the retrieved symbol addresses.
(https://repo.or.cz/openocd.git/blob/HEAD:/src/rtos/FreeRTOS.c#l60)

Unfortunately the specific offsets inside the structures can change based on
compilation options of FreeRTOS.

If GDB could report the information it already has from the debug information
(as shown in `ptype /o`) via its remote protocol then the logic in OpenOCD
could be more robust.

Ideas:

While a simple mapping of (structure name, member name) -> (offset) mapping
would be enough for my usecase it would probably be better to report more data.

* type of a typename (struct, union)
* total size of the type
* all members of the type including their own type and offsets

Example:

# request info for type "foo_t"
IN  qType:666f6f5f74
# "foo_t" is a struct of size 32 and members:
# * bar_t bar at offset 0
# * baz_t baz at offset 16
OUT qType:666f6f5f74:struct:32:0;6261725f74;626172,16;62617a5f74;62617a

(Or some XML equivalent)

Is this something that would fit into gdb?

Regards,
Thomas