[gdb/testsuite] Limit default_target_compile override

[gdb/testsuite] Limit default_target_compile override

The file lib/future.exp contains an override of dejagnu's
default_target_compile.

The override is activated if dejagnu's default_target_compile is missing
support for one or more languages.

However, if the override is activated, it's active for all languages.

This unnecessarily extends the scope of potential problems in the override to
languages that don't need the override.

Fix this by limiting the scope of the override.

Also add a note stating for which languages the override is active, as a
reminder that support for those languages needs to be ported to dejagnu.  With
my system dejagnu 1.6.1, as well as with current dejagnu trunk, that gives us:
...
NOTE: Dejagnu's default_target_compile is missing support for Go, using \
  local override
NOTE: Dejagnu's default_target_compile is missing support for Rust, using \
  local override
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-06-19  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_note): New proc.
	* lib/future.exp (gdb_default_target_compile_1): Factor out of ...
	(gdb_default_target_compile): ... here.  Only call
	gdb_default_target_compile_1 if use_gdb_compile(<lang>) is set.
	(use_gdb_compile): Change to array.
	(toplevel): Update sets of use_gdb_compile to specify language.
	Warn about default_target_compile override.  Store dejagnu's version
	of default_target_compile in dejagnu_default_target_compile.