[gdb/tui] Fix left margin in disassembly window

[gdb/tui] Fix left margin in disassembly window

With a hello world a.out, and maint set tui-left-margin-verbose on, we have
this disassembly window:
...
┌───────────────────────────────────────────────────────────┐
│___ 0x555555555149 <main>           endbr64                │
│___ 0x55555555514d <main+4>         push   %rbp            │
│___ 0x55555555514e <main+5>         mov    %rsp,%rbp       │
│B+> 0x555555555151 <main+8>         lea    0xeac(%rip),%rax│
│___ 0x555555555158 <main+15>        mov    %rax,%rdi       │
...
Note the space between "B+>" and 0x555555555151.  The space shows that a bit
of the left margin is not written, which is a problem because that location is
showing a character previously written, which happens to be a space, but also
may be something else, for instance a '[' as reported in PR tui/30325.

The problem is caused by confusion about the meaning of:
...
 #define TUI_EXECINFO_SIZE 4
...

There's the meaning of defining the size of this zero-terminated char array:
...
      char element[TUI_EXECINFO_SIZE];
...
which is used to print the "B+>" bit, which is 3 chars wide.

And there's the meaning of defining part of the size of the left margin:
...
  int left_margin () const
  { return 1 + TUI_EXECINFO_SIZE + extra_margin (); }
...
where it represents 4 chars.

The discrepancy between the two causes the space between "B+>" and
"0x555555555151".

Fix this by redefining TUI_EXECINFO_SIZE to 3, and using:
...
      char element[TUI_EXECINFO_SIZE + 1];
...
such that we have:
...
|B+>0x555555555151 <main+8>         lea    0xeac(%rip),%rax │
...

This changes the layout of the disassembly window back to what it was before
commit 9e820dec13e ("Use a curses pad for source and disassembly windows"),
the commit that introduced the PR30325 regression.

This also changes the source window from:
...
│___000005__{                                               |
...
to:
...
│___000005_{                                                |
...

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30325

Approved-By: Tom Tromey <tom@tromey.com>