gdb: Add switch to disable DWARF stack unwinders

gdb: Add switch to disable DWARF stack unwinders

Add a maintenance command to disable the DWARF stack unwinders.
Normal users would not need this feature, but it is useful to allow
extended testing of fallback stack unwinding strategies, for example,
prologue scanners.

This is a partial implementation of the idea discussed in pr gdb/8434,
which talks about a generic ability to disable any frame unwinder.

Being able to arbitrarily disable any frame unwinder would be a more
complex patch, and I was unsure how useful such a feature would really
be, however, I can see (and have) a real need to disable DWARF
unwinders.  That's why this patch only targets that specific set of
unwinders.

If in the future we find ourselves adding more switches to disable
different unwinders, then we should probably move to a more generic
solution, and remove this patch.

gdb/ChangeLog:

	* dwarf2-frame-tailcall.c (tailcall_frame_sniffer): Exit early if
	DWARF unwinders are disabled.
	* dwarf2-frame.c: Add dwarf2read.h include.
	(dwarf2_frame_sniffer): Exit early if DWARF unwinders are
	disabled.
	(dwarf2_frame_unwinders_enabled_p): Define.
	(show_dwarf_unwinders_enabled_p): New function.
	(_initialize_dwarf2_frame): Register switch to control DWARF
	unwinder use.
	* dwarf2-frame.h (dwarf2_frame_unwinders_enabled_p): Declare.
	* dwarf2read.c (set_dwarf_cmdlist): Remove static keyword.
	(show_dwarf_cmdlist): Remove static keyword.
	* dwarf2read.h (set_dwarf_cmdlist): Declare.
	(show_dwarf_cmdlist): Declare.
	* NEWS: Document new feature.

gdb/doc/ChangeLog:

	* gdb.texinfo (Maintenance Commands): Add description of
	maintenance command to control dwarf unwinders.

gdb/testsuite/ChangeLog:

	* gdb.base/maint.exp: Add check that dwarf unwinders control flag
	is visible.