Make sure a copy_insn_closure is available when we have a match in copy_insn_closure_...

Make sure a copy_insn_closure is available when we have a match in copy_insn_closure_by_addr

PR gdb/29272

Investigating PR29272, it was mentioned a particular test used to work on
GDB 10, but it started failing with GDB 11 onwards. I tracked it down to
some displaced stepping improvements on commit
187b041e2514827b9d86190ed2471c4c7a352874.

In particular, one of the corner cases using copy_insn_closure_by_addr got
silently broken. It is hard to spot because it doesn't have any good tests
for it, and the situation is quite specific to the Arm target.

Essentially, the change from the displaced stepping improvements made it so
we could still invoke copy_insn_closure_by_addr correctly to return the
pointer to a copy_insn_closure, but it always returned nullptr due to
the order of the statements in displaced_step_buffer::prepare.

The way it is now, we first write the address of the displaced step buffer
to PC and then save the copy_insn_closure pointer.

The problem is that writing to PC for the Arm target requires figuring
out if the new PC is thumb mode or not.

With no copy_insn_closure data, the logic to determine the thumb mode
during displaced stepping doesn't work, and gives random results that
are difficult to track (SIGILL, SIGSEGV etc).

Fix this by reordering the PC write in displaced_step_buffer::prepare
and, for safety, add an assertion to
displaced_step_buffer::copy_insn_closure_by_addr so GDB stops right
when it sees this invalid situation. If this gets broken again in the
future, it will be easier to spot.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29272

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