[gdb/testsuite] Fix gdb.tui/tui-layout-asm.exp with -m32

[gdb/testsuite] Fix gdb.tui/tui-layout-asm.exp with -m32

When running test-case gdb.tui/tui-layout-asm.exp with target board
unix/-m32, we run into:
...
FAIL: gdb.tui/tui-layout-asm.exp: scroll to end of assembler (scroll failed)
...

Comparing screen dumps (edited a bit to fit column width) before:
...
 0 +--------------------------------------------------------------------+
 1 | 0x8049194 <__libc_csu_init+68>  call   *-0x104(%ebp,%esi,4)        |
 2 | 0x804919b <__libc_csu_init+75>  add    $0x1,%esi                   |
 3 | 0x804919e <__libc_csu_init+78>  add    $0x10,%esp                  |
 4 | 0x80491a1 <__libc_csu_init+81>  cmp    %esi,%ebx                   |
 5 | 0x80491a3 <__libc_csu_init+83>  jne    0x8049188 <__...>           |
 6 | 0x80491a5 <__libc_csu_init+85>  add    $0xc,%esp                   |
 7 | 0x80491a8 <__libc_csu_init+88>  pop    %ebx                        |
 8 | 0x80491a9 <__libc_csu_init+89>  pop    %esi                        |
 9 | 0x80491aa <__libc_csu_init+90>  pop    %edi                        |
10 | 0x80491ab <__libc_csu_init+91>  pop    %ebp                        |
11 | 0x80491ac <__libc_csu_init+92>  ret                                |
12 | 0x80491ad                           lea    0x0(%esi),%esi          |
13 | 0x80491b0 <__libc_csu_fini>     ret                                |
14 +--------------------------------------------------------------------+
...
and after:
...
 0 +--------------------------------------------------------------------+
 1 | 0x804919b <__libc_csu_init+75>          add    $0x1,%esi           |
 2 | 0x804919e <__libc_csu_init+78>          add    $0x10,%esp          |
 3 | 0x80491a1 <__libc_csu_init+81>          cmp    %esi,%ebx           |
 4 | 0x80491a3 <__libc_csu_init+83>          jne    0x8049188 <__...>   |
 5 | 0x80491a5 <__libc_csu_init+85>          add    $0xc,%esp           |
 6 | 0x80491a8 <__libc_csu_init+88>          pop    %ebx                |
 7 | 0x80491a9 <__libc_csu_init+89>          pop    %esi                |
 8 | 0x80491aa <__libc_csu_init+90>          pop    %edi                |
 9 | 0x80491ab <__libc_csu_init+91>          pop    %ebp                |
10 | 0x80491ac <__libc_csu_init+92>          ret                        |
11 | 0x80491ad                                   lea    0x0(%esi),%esi  |
12 | 0x80491b0 <__libc_csu_fini>             ret                        |
13 | 0x80491b1 <__x86.get_pc_thunk.bp>       mov    (%esp),%ebp         |
14 +--------------------------------------------------------------------+
...
it seems the mismatch comes from the extra indentation forced by the longer
<__x86.get_pc_thunk.bp> that was scrolled in.

Fix this by ignoring whitespace when comparing scrolled lines.

Tested on x86_64-linux, using -m64 and -m32.

gdb/testsuite/ChangeLog:

2021-02-06  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26922
	* gdb.tui/tui-layout-asm.exp: Ignore whitespace mismatches when
	scrolling.