c++: don't warn about internal interference sizes

Most any compilation on ARM/AArch64 was warning because the default L1 cache
line size of 32B was smaller than the default
std::hardware_constructive_interference_size of 64B.  This is mostly due to
inaccurate --param l1-cache-line-size, but it's not helpful to complain to a
user that didn't set the values.

gcc/cp/ChangeLog:

	* decl.c (cxx_init_decl_processing): Only warn about odd
	interference sizes if they were specified with --param.