gdb: rename displaced_step_fixup to displaced_step_finish

gdb: rename displaced_step_fixup to displaced_step_finish

This is a preparatory patch to reduce a little bit the diff size of the
main patch later in this series.  It renames the displaced_step_fixup
function in infrun.c to displaced_step_finish.

The rationale is to better differentiate the low and high level
operations.

We first have the low level operation of writing an instruction to a
displaced buffer, called "copy_insn".  The mirror low level operation to
fix up the state after having executed the instruction is "fixup".  The
high level operation of preparing a thread for a displaced step (which
includes doing the "copy_insn" and some more bookkeeping) is called
"prepare" (as in displaced_step_prepare).  The mirror high level
operation to cleaning up after a displaced step (which includes doing
the "fixup" and some more bookkeeping) is currently also called "fixup"
(as in displaced_step_fixup), just like the low level operation.

I think that choosing a different name for the low and high level
cleanup operation makes it clearer, hence "finish".

gdb/ChangeLog:

	* infrun.c (displaced_step_fixup): Rename to...
	(displaced_step_finish): ... this, update all callers.

Change-Id: Id32f48c1e2091d09854c77fcedcc14d2519957a2