arm: irq: Disable clang warning on VFP clobbering

Clang warns the user whenever an ISR invokes a function that is not
marked with the `interrupt` attribute. This applies to the whole call
chain, which makes it unfeasible to add the attribute to add it to all
functions.

Instead, disable the clang warning just like it was done for the
equivalent GCC one in 5b3f8009.

See also:
https://reviews.llvm.org/D28820



Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>