gdb: print backtrace for internal error/warning

gdb: print backtrace for internal error/warning

This commit builds on previous work to allow GDB to print a backtrace
of itself when GDB encounters an internal-error or internal-warning.
This fixes PR gdb/26377.

There's not many places where we call internal_warning, and I guess in
most cases the user would probably continue their debug session.  And
so, in order to avoid cluttering up the output, by default, printing
of a backtrace is off for internal-warnings.

In contrast, printing of a backtrace is on by default for
internal-errors, as I figure that in most cases hitting an
internal-error is going to be the end of the debug session.

Whether a backtrace is printed or not can be controlled with the new
settings:

  maintenance set internal-error backtrace on|off
  maintenance show internal-error backtrace

  maintenance set internal-warning backtrace on|off
  maintenance show internal-warning backtrace

Here is an example of what an internal-error now looks like with the
backtrace included:

  (gdb) maintenance internal-error blah
  ../../src.dev-3/gdb/maint.c:82: internal-error: blah
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  ----- Backtrace -----
  0x5c61ca gdb_internal_backtrace_1
  	../../src.dev-3/gdb/bt-utils.c:123
  0x5c626d _Z22gdb_internal_backtracev
  	../../src.dev-3/gdb/bt-utils.c:165
  0xe33237 internal_vproblem
  	../../src.dev-3/gdb/utils.c:393
  0xe33539 _Z15internal_verrorPKciS0_P13__va_list_tag
  	../../src.dev-3/gdb/utils.c:470
  0x1549652 _Z14internal_errorPKciS0_z
  	../../src.dev-3/gdbsupport/errors.cc:55
  0x9c7982 maintenance_internal_error
  	../../src.dev-3/gdb/maint.c:82
  0x636f57 do_simple_func
  	../../src.dev-3/gdb/cli/cli-decode.c:97
   .... snip, lots more backtrace lines ....
  ---------------------
  ../../src.dev-3/gdb/maint.c:82: internal-error: blah
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n) y

  This is a bug, please report it.  For instructions, see:
  <https://www.gnu.org/software/gdb/bugs/>.

  ../../src.dev-3/gdb/maint.c:82: internal-error: blah
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Create a core file of GDB? (y or n) n

My hope is that this backtrace might make it slightly easier to
diagnose GDB issues if all that is provided is the console output, I
find that we frequently get reports of an assert being hit that is
located in pretty generic code (frame.c, value.c, etc) and it is not
always obvious how we might have arrived at the assert.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26377