gdbserver: refactor the definition and uses of supply_regblock

gdbserver: refactor the definition and uses of supply_regblock

The supply_regblock function takes a pointer to a buffer as an
argument and implements two different behavior based on the pointer
being null.  There are two cases where we pass nullptr, all in
tracepoint.cc, where we are essentially doing a reset on the regcache.

In fast_tracepoint_ctx::regcache, register_status array does not
even exist.  Hence, that use simply boils down to zeroing of register
data.  Do this at the time of creating the buffer and remove the call
to supply_regblock.

In fetch_traceframe_registers, inline the use with a call to `reset`.

Hence, there are no more cases left, where a nullptr would be passed
to supply_regblock.  Assert that the buffer argument is non-null and
simplify the implementation.

Approved-By: Simon Marchi <simon.marchi@efficios.com>