PowerPC: regression fix for reverse-finish command.

The recent commit:

  commit 2a8339b7
  Author: Carl Love <cel@us.ibm.com>
  Date:   Thu Mar 9 16:10:18 2023 -0500

   PowerPC: fix for gdb.reverse/finish-precsave.exp and gdb.reverse/finish-reverse.exp

   PPC64 multiple entry points, a normal entry point and an alternate entry
   point.  The alternate entry point is to setup the Table of Contents (TOC)
   register before continuing at the normal entry point.  When the TOC is
   already valid, the normal entry point is used, this is typically the case.
   The alternate entry point is typically referred to as the global entry
   point (GEP) in IBM.  The normal entry point is typically referred to as
   the local entry point (LEP).
     .....

Is causing regression failures on on PowerPC platforms.  The regression
failures are in tests:

  gdb.reverse/finish-precsave.exp
  gdb.btrace/tailcall.exp
  gdb.mi/mi-reverse.exp
  gdb.btrace/step.exp
  gdb.reverse/until-precsave.exp
  gdb.reverse/finish-reverse.exp
  gdb.btrace/tailcall-only.exp

The issue is in gdb/infcmd.c, function finish_command.  The value of the
two new variables ALT_ENTRY_POINT and ENTRY_POINT are being initializezed
to SAL.PC.  However, SAL has just been declared.  The value of SAL.PC is
zero at this point.  The intialization of ALT_ENTRY_POINT and ENTRY_POINT
needs to be after the initialization of SAL.

This patch moves the initialization of ALT_ENTRY_POINT and ENTRY_POINT
variables to fix the regression failures.

The patch has been tested on Power10 and on X86.