Introduce show_debug_regs

Introduce show_debug_regs

This commit adds a new global flag show_debug_regs to common-debug.h
to replace the flag debug_hw_points used by gdbserver and by the
Linux x86 and AArch64 ports, and to replace the flag maint_show_dr
used by the Linux MIPS port.

Note that some debug printing in the AArch64 port was enabled only if
debug_hw_points > 1 but no way to set debug_hw_points to values other
than 0 and 1 was provided; that code was effectively dead.  This
commit enables all debug printing if show_debug_regs is nonzero, so
the AArch64 output will be more verbose than previously.

gdb/ChangeLog:

	* common/common-debug.h (show_debug_regs): Declare.
	* common/common-debug.c (show_debug_regs): Define.
	* aarch64-linux-nat.c (debug_hw_points): Don't define.  Replace
	all uses with show_debug_regs.  Replace all uses that considered
	debug_hw_points as a multi-value integer with straight boolean
	uses.
	* x86-nat.c (debug_hw_points): Don't define.  Replace all uses
	with show_debug_regs.
	* nat/x86-dregs.c (debug_hw_points): Don't declare.  Replace
	all uses with show_debug_regs.
	* mips-linux-nat.c (maint_show_dr): Don't define.  Replace all
	uses with show_debug_regs.

gdb/gdbserver/ChangeLog:

	* server.h (debug_hw_points): Don't declare.
	* server.c (debug_hw_points): Don't define.  Replace all uses
	with show_debug_regs.
	* linux-aarch64-low.c (debug_hw_points): Don't define.  Replace
	all uses with show_debug_regs.