gdb/tui: Fix 'layout asm' before the inferior has started

gdb/tui: Fix 'layout asm' before the inferior has started

Currently if a user starts the tui with 'layout asm' then they will be
presented with the 'src' layout.

What happens is:

  1. Layout command enables TUI, selecting the SRC layout by default.

  2. As part of tui_enable we call tui_display_main, which calls
     tui_get_begin_asm_address, which calls
     set_default_source_symtab_and_line.  This changes core GDBs
     current symtab and line, which triggers a call to the symtab
     changed hook tui_symtab_changed, which sets the flag
     from_source_symtab.

  3. Back in the layout command, the layout is changed from SRC to
     ASM.  After this the layout command completes and we return to
     core GDB which prints the prompt, however...

  4. The before prompt hook is called which sees the
     from_source_symtab flag is set and forces the SRC window to be
     displayed.  This switches us back to SRC view.

The solution I propose here is to delay installing the hooks into core
GDB until after we have finished setting up the tui and selecting the
default frame to view.  In this way we effectively ignore the first
symtab changed event triggered when making main the default symtab.

gdb/ChangeLog:

	* tui/tui.c (tui_enable): Register tui hooks after calling
	tui_display_main.

gdb/testsuite/ChangeLog:

	* gdb.tui/tui-layout-asm.exp: New file.

Change-Id: I858ab81a17ffb4aa72deb3f36c3755228a9c9d9a