gdb: unify parts of the Linux and FreeBSD core dumping code

While reviewing the Linux and FreeBSD core dumping code within GDB for
another patch series, I noticed that the code that collects the
registers for each thread and writes these into ELF note format is
basically identical between Linux and FreeBSD.

This commit merges this code and moves it into the gcore.c file,
which seemed like the right place for generic writing a core file
code.

The function find_signalled_thread is moved from linux-tdep.c despite
not being shared.  A later commit will make use of this function.

There are a couple of minor changes to the FreeBSD target after this
commit, but I believe that these are changes for the better:

(1) For FreeBSD we always used to record the thread-id in the core file by
using ptid_t.lwp ().  In contrast the Linux code did this:

    /* For remote targets the LWP may not be available, so use the TID.  */
    long lwp = ptid.lwp ();
    if (lwp == 0)
      lwp = ptid.tid ();

Both target now do this:

    /* The LWP is often not available for bare metal target, in which case
       use the tid instead.  */
    if (ptid.lwp_p ())
      lwp = ptid.lwp ();
    else
      lwp = ptid.tid ();

Which is equivalent for Linux, but is a change for FreeBSD.  I think
that all this means is that in some cases where GDB might have
previously recorded a thread-id of 0 for each thread, we might now get
something more useful.

(2) When collecting the registers for Linux we collected into a zero
initialised buffer.  By contrast on FreeBSD the buffer is left
uninitialised.  In the new code the buffer is always zero initialised.
I suspect once the registers are copied into the buffer there's
probably no gaps left so this makes no difference, but if it does then
using zeros rather than random bits of GDB's memory is probably a good
thing.

Otherwise, there should be no other user visible changes after this
commit.

Tested this on x86-64/GNU-Linux and x86-64/FreeBSD-12.2 with no
regressions.

gdb/ChangeLog:

	* Makefile.in (HFILES_NO_SRCDIR): Add corefile.h.
	* gcore.c (struct gcore_collect_regset_section_cb_data): Moved
	here from linux-tdep.c and given a new name.  Minor cleanups.
	(gcore_collect_regset_section_cb): Likewise.
	(gcore_collect_thread_registers): Likewise.
	(gcore_build_thread_register_notes): Likewise.
	(gcore_find_signalled_thread): Likewise.
	* gcore.h (gcore_build_thread_register_notes): Declare.
	(gcore_find_signalled_thread): Declare.
	* fbsd-tdep.c: Add 'gcore.h' include.
	(struct fbsd_collect_regset_section_cb_data): Delete.
	(fbsd_collect_regset_section_cb): Delete.
	(fbsd_collect_thread_registers): Delete.
	(struct fbsd_corefile_thread_data): Delete.
	(fbsd_corefile_thread): Delete.
	(fbsd_make_corefile_notes): Call
	gcore_build_thread_register_notes instead of the now deleted
	FreeBSD code.
	* linux-tdep.c: Add 'gcore.h' include.
	(struct linux_collect_regset_section_cb_data): Delete.
	(linux_collect_regset_section_cb): Delete.
	(linux_collect_thread_registers): Delete.
	(linux_corefile_thread): Call
	gcore_build_thread_register_notes.
	(find_signalled_thread): Delete.
	(linux_make_corefile_notes): Call gcore_find_signalled_thread.