[gdb/testsuite] Handle older gcc in gdb.ada/import.exp

[gdb/testsuite] Handle older gcc in gdb.ada/import.exp

When running test-case gdb.ada/import.exp with gcc 7, most test fail:
...
FAIL: gdb.ada/import.exp: print imported_var_ada
FAIL: gdb.ada/import.exp: print local_imported_var
FAIL: gdb.ada/import.exp: print pkg.imported_var_ada
FAIL: gdb.ada/import.exp: print pkg.exported_var_ada
FAIL: gdb.ada/import.exp: print exported_var_ada
FAIL: gdb.ada/import.exp: gdb_breakpoint: set breakpoint at pkg.imported_func_ada
FAIL: gdb.ada/import.exp: gdb_breakpoint: set breakpoint at imported_func_ada
FAIL: gdb.ada/import.exp: gdb_breakpoint: set breakpoint at local_imported_func
...

When running with gcc 8 or 9, only 2 tests fail:
...
FAIL: gdb.ada/import.exp: gdb_breakpoint: set breakpoint at pkg.imported_func_ada
FAIL: gdb.ada/import.exp: gdb_breakpoint: set breakpoint at imported_func_ada
...

The test-case passes fully with gcc 10, 11, 12 and 13.

Debug info for pragma import seems to not have been supported before gcc 8, so
require that version.

The two FAILs with gcc 8 and 9 seem to be due to problems in debug info.  Add
an xfail for these.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>