Bug 509406 - FreeBSD 15 issues

Bug 509406 - FreeBSD 15 issues

This is the last of a series of fixes and bodges.

I now get the following results.

On arm64
== 793 tests, 1 stderr failure, 0 stdout failures, 0 stderrB failures, 0 stdoutB failures, 0 post failures ==
memcheck/tests/thread_alloca             (stderr)

(which is not a new failure) and

== 947 tests, 1 stderr failure, 0 stdout failures, 0 stderrB failures, 1 stdoutB failure, 0 post failures ==
gdbserver_tests/nlvgdbsigqueue           (stderr)
gdbserver_tests/nlvgdbsigqueue           (stdoutB)

(again a test that fails regularly)