testsuite: Add replay logging to GDBSERVER_DEBUG

testsuite: Add replay logging to GDBSERVER_DEBUG

Add "replay" to the list of GDBSERVER_DEBUG options.  This will
cause a gdbserver.replay file to be written to the test output
directory.

At the same time switch this to a comma separated list in order
to easily handle all possible options.

The replay log is created by GDB, but has been added to
GDBSERVER_DEBUG as it is only required for gdbserver tests. To
enable it, the gdb_debug_init is overridden to allow the additional
checking, before calling the original function.

gdb/testsuite/ChangeLog:

        * README (Testsuite Parameters): Add replay logging to
        GDBSERVER_DEBUG.
        (gdbserver,debug): Refer to GDBSERVER_DEBUG.
        * lib/gdbserver-support.exp (gdbserver_start): Treat gdbserverdebug
        as a comma separated list.
        (gdb_debug_init): Override procedure.