[gdb/tdep] Fix reverse execution of LDR(immediate) T4

When running test-case gdb.reverse/func-map-to-same-line.exp on arm-linux with
target board unix/-mthumb, we run into:
...
(gdb) reverse-step
func2 () at func-map-to-same-line.c:26
26	{
(gdb) FAIL: gdb.reverse/func-map-to-same-line.exp: \
  column_info_flag=column-info: step-test: reverse-step into func2
...

The FAIL is caused by incorrect recording of this insn:
...
4f6:   f85d 7b04       ldr.w   r7, [sp], #4
...

The insn updates the sp, but we don't record this:
...
$ gdb -q -batch func-map-to-same-line \
  -ex "b *func2+8" \
  -ex run \
  -ex record \
  -ex "set debug record 2" \
  -ex stepi
Breakpoint 1 at 0x4f6: file func-map-to-same-line.c, line 27.

Breakpoint 1, 0xaaaaa4f6 in func2 () at func-map-to-same-line.c:27
27	} /* END FUNC2 */
Process record: arm_process_record addr = 0xaaaaa4f6
Process record: add register num = 15 to record list.
Process record: record_full_arch_list_add 0xabc6c460.
Process record: add register num = 7 to record list.
Process record: record_full_arch_list_add 0xabc3b868.
Process record: add register num = 25 to record list.
...
[ Note that sp is r13, and we see here only r15 (pc), r7, and r25 (ps). ]

The problem is that the specific insn, an LDR(immediate) T4, is not handled in
thumb2_record_ld_word.

Fix this by detecting the insn in thumb2_record_ld_word, and recording the
updated base register.

Tested on arm-linux.

Reported-By: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Approved-By: Luis Machado <luis.machado@arm.com>

PR tdep/31278
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31278