Fix gnu folding constant

[thughes]

When building with clang it warns:

    tests/kernel/common/src/pow2.c:30:6: error: variable length array folded
    to constant array as an extension [-Werror,-Wgnu-folding-constant]
    char static_array1[Z_POW2_CEIL(1)];
         ^

We want the variable length array folded into a constant array, so disable the warning.

Depends on #84063

[npitre]

This is wrong.

Why would clang warn when the constant 1 is used? Why not with 2 or 3?
Why warn at all in the first place?

If this clang warning isn't useful, perhaps it should simply be disabled
altogether and not papered over each time it comes up. Or perhaps a clang
specific Z_POW2_CEIL() implementation is needed. But as presented this
makes no sense to me.

[npitre]

My previous concerns still stand.

Having to surround code with IN_WARNING_GNU_FOLDING_CONSTANT) and
TOOLCHAIN_ENABLE_CLANG_WARNING(TOOLCHAIN_WARNING_GNU_FOLDING_CONSTANT) is
rather unwieldy.

Question:

• Why in the test is it necessary for only one line? Why not for the other
  lines that use the same construct?

• Is this particular warning any useful in the first place?

I'm open to be convinced otherwise, but at the moment I think this
particular warning ought to be disabled globally for the entire tree.