Reimplement "tui reg" command

Reimplement "tui reg" command

This reimplements the low-level layout function that is used by the
"tui reg" command.  Now it simply calls into the existing "layout"
command, though this will be changed again in a subsequent patch.  The
rationale for this patch is that it makes it simpler to remove
"enum tui_layout_type".

gdb/ChangeLog
2020-02-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.c (tui_reg_layout): Remove.
	(tui_reg_command): Use tui_regs_layout.
	* tui/tui-layout.h (tui_reg_command): Declare.
	* tui/tui-layout.c (tui_reg_command): New function.

Change-Id: I0ca6884e2967005e7d3fbf5f13a0ac8f9c3298cf