gdb: fix GDB_DEBUG and GDBSERVER_DEBUG Makefile variables

gdb: fix GDB_DEBUG and GDBSERVER_DEBUG Makefile variables

The gdb/testsuite/README file documents GDB_DEBUG and GDBSERVER_DEBUG
flags, which can be passed to make in order to enable debugging within
GDB or gdbserver respectively.

However, when I do:

  make check-gdb GDB_DEBUG=infrun

I don't see the corresponding debug feature within GDB being enabled.
Nor does:

  make check-gdb GDBSERVER_DEBUG=debug  \
       RUNTESTFLAGS="--target_board=native-extended-gdbserver"

Appear to enable gdbserver debugging.

I tracked this down to the GDB_DEBUG and GDBSERVER_DEBUG flags being
missing from the TARGET_FLAGS_TO_PASS variable in gdb/Makefile.  This
variable already contains lots of testing related flags, like
RUNTESTFLAGS and TESTS, so I think it makes sense to add GDB_DEBUG and
GDBSERVER_DEBUG here too.

With this done, this debug feature is now working as expected.

Approved-By: Tom Tromey <tom@tromey.com>