[gdb/testsuite] Fix gdb.base/catch-syscall.exp without --enable-targets

[gdb/testsuite] Fix gdb.base/catch-syscall.exp without --enable-targets

When doing a gdb build without --enable-targets, I run into:
...
(gdb) UNSUPPORTED: gdb.base/catch-syscall.exp: multiple targets: \
  s390:31-bit vs s390:64-bit: set architecture s390:64-bit
delete breakpoints^M
(gdb) info breakpoints^M
No breakpoints or watchpoints.^M
(gdb) break -qualified main^M
No symbol table is loaded.  Use the "file" command.^M
Make breakpoint pending on future shared library load? (y or [n]) n^M
(gdb) FAIL: gdb.base/catch-syscall.exp: gdb_breakpoint: set breakpoint at main
...

The problem is that due to recent commit e21d8399303 ("[gdb/testsuite] Remove
target limits in gdb.base/catch-syscall.exp") "clean_restart $binfile" no
longer is called at the end of test_catch_syscall_multi_arch.

Fix this by moving "clean_restart $binfile" back to
test_catch_syscall_multi_arch.

Tested on x86_64-linux.