btrace: pretend we're not replaying when generating a core file

btrace: pretend we're not replaying when generating a core file

When generating a core file using the "generate-core-file" command while
replaying with the btrace record target, we won't be able to access all
registers and all memory.  This leads to the following assertion:

    gdb/regcache.c:1034: internal-error: regcache_raw_supply: Assertion `regnum >= 0 && regnum < regcache->descr->nr_raw_registers' failed.
    A problem internal to GDB has been detected,
    further debugging may prove unreliable.
    Quit this debugging session? (y or n) FAIL: gdb.btrace/gcore.exp: generate-core-file core (GDB internal error)
    Resyncing due to internal error.

Pretend that we are not replaying while generating a core file.  This will
forward fetch and store registers as well as xfer memory calls to the target
beneath.

gdb/
	* record-btrace.c (record_btrace_generating_corefile)
	(record_btrace_prepare_to_generate_core)
	(record_btrace_done_generating_core): New.
	(record_btrace_xfer_partial, record_btrace_fetch_registers)
	(record_btrace_store_registers, record_btrace_prepare_to_store):
	Forward request when generating a core file.
	(record_btrace_open): Set record_btrace_generating_corefile to zero.
	(init_record_btrace_ops): Set to_prepare_to_generate_core and
	to_done_generating_core.

testsuite/
	* gdb.btrace/gcore.exp: New.