CMake: enable -ffp-exceptions-behavior=strict when compiling with clang

CMake: enable -ffp-exceptions-behavior=strict when compiling with clang

We have to ensure that we emit floating-point instructions in debug mode
that preserve the occurence of floating-point exceptions and don't
introduce new ones. gcc plays nicely in this regard by enabling
`-ftrapping-math` per default, at least for the level of optimization we
have in debug mode. clang however is more aggressive and assumes that it
can optimize code disregarding precise floating-point exception
semantics.

We thus set `-ffp-exceptions-behavior=strict` in debug mode to ensure
that our testsuite doesn't run into false positive floating-point
exceptions. See https://github.com/dealii/dealii/issues/15496