[gdb/testsuite] Handle unsupported catch syscall

On riscv64-linux, I run into:
...
Expecting: ^(catch syscall[^M
]+)?((&.*)*.*~"Catchpoint 5 .*\\n".*=breakpoint-created,bkpt=\{number="5",type="catchpoint".*\}.*\n\^done[^M
]+[(]gdb[)] ^M
[ ]*)
catch syscall^M
&"catch syscall\n"^M
&"The feature 'catch syscall' is not supported on this architecture yet.\n"^M
^error,msg="The feature 'catch syscall' is not supported on this architecture yet."^M
(gdb) ^M
FAIL: gdb.mi/mi-breakpoint-changed.exp: test_insert_delete_modify: catch syscall (unexpected output)
...

Fix this by:
- factoring out proc supports_catch_syscall out of gdb.base/catch-syscall.exp,
  and
- using it in gdb.mi/mi-breakpoint-changed.exp.

Tested on x86_64-linux and riscv64-linux.

Approved-By: Andrew Burgess <aburgess@redhat.com>