[gdb/testsuite] Fix gdb.ada/verylong.exp on 32-bit target

[gdb/testsuite] Fix gdb.ada/verylong.exp on 32-bit target

In an aarch32-linux chroot on an aarch64-linux system, I run into:
...
(gdb) print x^M
$1 = 9223372036854775807^M
(gdb) FAIL: gdb.ada/verylong.exp: print x
...

A passing version on aarch64-linux looks like:
...
(gdb) print x^M
$1 = 170141183460469231731687303715884105727^M
(gdb) PASS: gdb.ada/verylong.exp: print x
...

The difference is caused by the size of the type Long_Long_Long_Integer, which
is:
- a 128-bit signed on 64-bit targets, and
- a 64-bit signed on 32-bit target.

Fix this by detecting the size of the Long_Long_Long_Integer type, and
handling it.

Tested on aarch64-linux and aarch32-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR testsuite/31574
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31574

[1] https://gcc.gnu.org/onlinedocs/gnat_rm/Implementation-Defined-Characteristics.html