gdb_is_target_native -> gdb_protocol_is_native

gdb_is_target_native -> gdb_protocol_is_native

gdb_is_target_native uses "maint print target-stack", which is
unnecessary when checking whether gdb_protocol is empty would do.
Checking gdb_protocol is more efficient, and can be done before
starting GDB and running to main, unlike gdb_is_target_native.

This adds a new gdb_protocol_is_native procedure, and uses it in place
of gdb_is_target_native.

At first, I thought that we'd end up with a few testcases needing to
use gdb_is_target_native still, especially multi-target tests that
connect to targets different from the default board target, but no,
actually all uses of gdb_is_target_native could be converted.
gdb_is_target_native will be eliminated in a following patch.

In some spots, we no longer need to defer the check until after
starting GDB, so the patch adjusts accordingly.

Change-Id: Ia706232dbffac70f9d9740bcb89c609dbee5cee3
Approved-By: Tom Tromey <tom@tromey.com>