Disable asan sigaltstack for select clang unit tests

Apparently LLVM's internal sigaltstack usage does not play well
together with the sanitizers, see [1]. This leads to failed tests
on our CI. Setting use_sigaltstack=0 as an ASAN_OPTION fixes this
issue, which this patch here does.

Sadly, we have to jump through some hoops to achieve this though.
On one hand, we want to extend the ASAN_OPTIONS, not overwrite it.
As such, we can't just set the ctest ENVIRONMENT property, as the
CI sets a bunch of custom flags in the ASAN_OPTIONS env var. Users
might have their own set of values there too. Once we can rely on
CMake 3.22 or newer, we might use ENVIRONMENT_MODIFICATION.

Furthermore, I could not get __asan_default_options() to work
with GCC.

So, instead for now we use some dirty hackery: We check whether the
use_sigaltstack option is set in ASAN_OPTIONS. If not, we set it
explicitly to 0 and then execve the test anew. This way we can be
sure that the environment gets picked up properly.

[1]: https://github.com/google/sanitizers/issues/849