gdb/testsuite: Skip dump ihex for 64-bit address in gdb.base/dump.exp

gdb/testsuite: Skip dump ihex for 64-bit address in gdb.base/dump.exp

(1) Description of problem

In the current code, when execute the following test on LoongArch:

$make check-gdb TESTS="gdb.base/dump.exp"
```
FAIL: gdb.base/dump.exp: dump array as value, intel hex
FAIL: gdb.base/dump.exp: dump struct as value, intel hex
FAIL: gdb.base/dump.exp: dump array as memory, ihex
FAIL: gdb.base/dump.exp: dump struct as memory, ihex

```
These tests passed on the X86_64,

(2) Root cause

On LoongArch, variable intarray address 0x120008068 out of range for IHEX,
so dump ihex test failed.

gdb.base/dump.exp has the following code to check 64-bit address

```
 # Check the address of a variable.  If it is bigger than 32-bit,
 # assume our target has 64-bit addresses that are not supported by SREC,
 # IHEX and TEKHEX.  We skip those tests then.
 set max_32bit_address "0xffffffff"
 set data_address [get_hexadecimal_valueof "&intarray" 0x100000000]
 if {${data_address} > ${max_32bit_address}} {
    set is64bitonly "yes"
}
```

We check the "&intarray" on different target as follow:

```
$gdb gdb/testsuite/outputs/gdb.base/dump/dump
...
(gdb) start
...

On X86_64:
(gdb) print /x &intarray
$1 = 0x404060

On LoongArch:
(gdb) print /x &intarray
$1 = 0x120008068
```
The variable address difference here is due to the link script
of linker.

```
On X86_64:
$ld --verbose
...
PROVIDE (__executable_start = SEGMENT_START("text-segment", 0x400000));
. = SEGMENT_START("text-segment", 0x400000) + SIZEOF_HEADERS;

On LoongArch:
$ld --verbose
...
PROVIDE (__executable_start = SEGMENT_START("text-segment", 0x120000000));
. = SEGMENT_START("text-segment", 0x120000000) + SIZEOF_HEADERS;

```

(3) How to fix

Because 64-bit variable address out of range for IHEX, it's not an
functional problem for LoongArch. Refer to the handling of 64-bit
targets in this testsuite, use the "is64bitonly" flag to skip those
tests for the target has 64-bit addresses.

Signed-off-by: Hui Li <lihui@loongson.cn>
Approved-By: Tom Tromey <tom@tromey.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>