testsuite: Correct check_effective_target_hwaddress_exec

This test should ensure that we can compile with hwasan, that such a compiled
binary runs as expected, *and* that we're running on a kernel which implements
the capability to ignore the top bytes of pointers in syscalls.

It was expected that a basic test of `int main(void) { return 0; }` would check
this, since there is a check called during `__hwasan_init` in libhwasan to
ensure that the kernel we're running on provides a `prctl` to request the
relaxed ABI.

Unfortunately, the check in libhwasan has a bug in it, and does not correctly
fail when the kernel feature is not around.  This means that check is not
automatically provided by the runtime.

The sanitizer runtime will be fixed but would like to install this fix here in
case fixing the library is not quick enough for the release (and so that people
running the testsuite do not see spurious errors in the meantime).

Tested by running testsuite on an AArch64 machine with and without the required
kernel.
Observed that the test does indeed fail when the kernel feature is unavailable
and pass when the feature is available.

Note that this test is directly targetting AArch64 by using `prctl` numbers
specific to it.  That's unfortunate, but once the runtime fix has gone in we
will be able to remove that requirement.

Ok for trunk?

gcc/testsuite/ChangeLog:

	* lib/hwasan-dg.exp (check_effective_target_hwaddress_exec): Fix
	check for correct kernel version.

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