package/gdb: fix build of gdbserver-only on the ARC gdb version

package/gdb: fix build of gdbserver-only on the ARC gdb version

The GDB version used on ARC is based on a recent gdb code base, post
gdb 9.2. This recent gdb code base, which pre-figures what will be in
gdb 10, has a significant change: gdbserver is not longer in
gdb/gdbserver, but at the top-level, and the mechanism to build
gdbserver only has changed. Due to this change, a build of ARC GDB for
gdbserver only fails with:

/bin/bash: line 0: cd: /opt/output/build/gdb-arc-2020.03-release-gdb/gdb/gdbserver: No such file or directory

This commit adjusts gdb.mk to support four cases:

 - "old" gdb, gdbserver only
 - "old" gdb, with the gdb debugger (and optionally gdbserver as well)
 - "new" gdb, gdbserver only
 - "new" gdb, with the gdb debugger (and optionally gdbserver as well)

A boolean GDB_GDBSERVER_TOPLEVEL is introduced to differentiate
between the old and new gdb, it is set to "y" for gdb versions that
have the gdbserver code at the top-level. For now, only the ARC
version sets it, but in the future, upstream gdb version 10 will also
have to set it.

Here is the behavior, for each case:

 (1) "old" gdb, gdbserver only

     We set GDB_SUBDIR to gdb/gdbserver, so only the configure script
     in this folder gets called.

     --enable-gdbserver --disable-gdb are passed in CONF_OPTS.

 (2) "old" gdb, with the gdb debugger (and optionally gdbserver as well)

     We set GDB_SUBDIR to build/, an empty directory which allows to
     do an out of tree build, which is mandatory for a full gdb build
     since gdb 9.x.

     --enable-gdb is passed in CONF_OPTS as well as --enable-gdbserver
     or --disable-gdbserver depending on whether gdbserver is enabled
     as well.

 (3) "new" gdb, gdbserver only

     We set GDB_SUBDIR to build/, an empty directory which allows to
     do an out of tree build, which is mandatory for a full gdb build
     since gdb 9.x.

     --enable-gdbserver --disable-gdb are passed in CONF_OPTS.

 (4) "new" gdb, with the gdb debugger (and optionally gdbserver as well)

     We set GDB_SUBDIR to build/, an empty directory which allows to
     do an out of tree build, which is mandatory for a full gdb build
     since gdb 9.x.

     --enable-gdb is passed in CONF_OPTS as well as --enable-gdbserver
     or --disable-gdbserver depending on whether gdbserver is enabled
     as well.

In addition to these changes, some related changes are done as well:

 - We re-enable building both gdb and gdbserver on ARC, as it works
   again.

 - We only pass --with-curses when curses is really provided, i.e when
   the full debugger is being built.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>