[gdb/testsuite] Check gnatmake version in gdb.ada/scalar_storage.exp

[gdb/testsuite] Check gnatmake version in gdb.ada/scalar_storage.exp

On a system with gcc 14.2.0 and gnatmake 13.3.0 I run into:
...
(gdb) PASS: gdb.ada/scalar_storage.exp: print V_LE
get_compiler_info: gcc-14-2-0
print V_BE^M
$2 = (value => 126, another_value => 12, color => red)^M
(gdb) FAIL: gdb.ada/scalar_storage.exp: print V_BE
...

The test-case contains a corresponding kfail:
...
 # This requires a compiler fix that is in GCC 14.
 if {[gcc_major_version] < 14} {
     setup_kfail "DW_AT_endianity on enum types" *-*-*
 }
...
which doesn't trigger because it checks the gcc version rather than the
gnatmake version.

Fix this by checking the gnatmake version instead.

Tested on aarch64-linux and x86_64-linux.