gdb/testsuite: return after reporting a test unsupported

In this commit:

  commit 8bcead69
  Date:   Tue May 23 11:25:01 2023 +0100

      gdb/testsuite: add test for core file with a 0 pid

a new test gdb.arch/core-file-pid0.exp was added.  This test includes
a pre-generated core file for x86-64 and for other architectures the
test reports 'unsupported'.

However, after reporting 'unsupported' the test failed to perform an
early return, so the test would then carry on and try to actually
perform the test, which resulted in some TCL errors.

Fix this by returning after reporting the test unsupported.