gdb/testsuite: Regenerate the testglue if it is not in

gdb/testsuite: Regenerate the testglue if it is not in

For running the  DejaGnu  tests,  some  esoteric  configurations
may require a testglue.   This,  for  instance,  is  true  about
testing ARC  targets  which  uses  its  own  DejaGnu  board  and
a simulator which does not support returning the program's  exit
code.  Therefore, for those  tests  that  use  "gdb_compile",  a
"gdb_tg.o"  file  is  compiled  and  linked   into   the   final
executable.

There  are  tests  that  invoke  "gdb_compile"  from   different
directories.   Let's  take  a   look   at   an   example   test:
gdb.base/fullname.exp.  The purpose of this  test  is  to  build
the executable from different directories (absolute vs. relative
vs.  other) and then check if gdb can handle setting breakpoints
accordingly.

When  "gdb_compile"  generates  the  "gdb_tg.o",  it  does   not
do it again  for  the  same  test.   Although  this  might  seem
efficient, it can lead to  problems  when  changing  directories
before the next compile:

  gdb compile failed, arc-elf32-gcc: error: gdb_tg.o:
  No such file or directory

This patch checks if the wrapper file ("gdb_tg.o") is  still  in
reach and if it is not, it will stimulate  the  regeneration  of
the wrapper.

It is worth mentioning that GCC's  DejaGnu  tests  handle  these
scenarios as well and they seem to be more efficient in doing so
by saving the library paths and manipulating them  if  necessary
[1].  However, for GDB tests, that  require  less  compilations,
I think the proposed solution should be fine compared to a  more
full fledged solution from GCC.  The glue file in  our  case  is
only 2 KiB.

Last but not least, I ran the x86_64 tests on an x86_64 host and
found no regression.

[1]
Avid  coders  may  look  for  "set_ld_library_path_env_vars"  in
gcc/testsuite/lib/target-libpath.exp.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (gdb_wrapper_init): Reset
	"gdb_wrapper_initialized" to 0 if "wrapper_file" does
	not exist.