Unify ctf_fetch_registers and tfile_fetch_registers

Functions ctf_fetch_registers and tfile_fetch_registers have some
duplicated code about guessing the PC in regcache.  Sometimes, we
may change one function and forget to update the other one, like this
https://www.sourceware.org/ml/gdb-patches/2014-01/msg00292.html

This patch is to move the duplicated code into a new function
tracefile_fetch_registers, and let both ctf_fetch_registers and
tfile_fetch_registers call it.

gdb:

2014-04-22  Yao Qi  <yao@codesourcery.com>

	* tracefile-tfile.c (tfile_fetch_registers): Move the bottom to ...
	* tracefile.c (tracefile_fetch_registers): ... it.  New function.
	* tracefile.h (tracefile_fetch_registers): Declare.
	* ctf.c (ctf_fetch_registers): Remove the bottom.  Call
	tracefile_fetch_registers.