Fix more bugs in gdb testglue wrapper handling

Fix more bugs in gdb testglue wrapper handling

In commit 24ac169ac5a918cd82b7485935f0c40a094c625e, this patch:

	2020-02-21  Shahab Vahedi  <shahab@synopsys.com>

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

attempted to fix problems finding the gdb test wrapper gdb_tg.o in
some tests that cd to some non-default directory by rebuilding also
the test wrapper in that directory.  This had the side-effect of
leaving these .o files in various places in the GDB source directory
tree.

Furthermore, while the tests that cd to some non-default directory
cannot run on remote host, the code that was added to probe for the
presence of the wrapper file was also specific to host == build.

This patch reverts the problematic parts of that commit and replaces
it with forcing use of an absolute (rather than relative) pathname to
the .o file for linking when host == build.

While debugging this patch, I also observed that use of the construct
"[info exists gdb_wrapper_file]" was not reliable for detecting when
that variable had been initialized by gdb_wrapper_init.  I rewrote
that so that the variable is always initialized and has a value of an
empty string when no wrapper file is needed.

2020-07-22  Sandra Loosemore  <sandra@codesourcery.com>

	gdb/testsuite/
	* lib/gdb.exp (gdb_wrapper_file, gdb_wrapper_flags):
	Initialize to empty string at top level.
	(gdb_wrapper_init): Revert check for file existence on build.
	Build the wrapper in its default place, not a build-specific
	location.  When host == build, make the pathname absolute.
	(gdb_compile): Delete leftover declaration of
	gdb_wrapper_initialized.  Check gdb_wrapper_file being an empty
	string instead of uninitialized.