cmake: disable warning on C++ 20

Compound assigments like x |= y to volatile qualified values are
deprecated in C++20.  This idiom is common in CMSIS headers for
manipulating peripheral registers.  Since these headers are core they
are likely to be included indirectly in C++ modules; since they are in
vendor-provided headers we can't "fix" them.

Disable the warning when building for C++.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>