Continuation of support for target description parameters

Continuation of support for target description parameters

With this commit, it's possible to declare parameters in a feature, and
generate C and XML feature files.

The regcache also supports using parameters for variable-size registers,
and they can be "unwound" from the sentinel frame.

There's a new gdbarch_fetch_tdesc_parameter hook to request the value of
a parameter from the architecture. Maybe a target hook will be needed
too, for remote debugging.

There's generic support for unwinding parameters from a frame, but for
now only the sentinel frame allows it.

Remote debugging a target which uses parameters in the target
description isn't yet supported.