meson: enable -Warray-bounds and -fstrict-flex-arrays

meson: enable -Warray-bounds and -fstrict-flex-arrays

-fstrict-flex-arrays means that the compiler doesn't have to assume that any
trailing array is a flex array. I.e. unless the array is declared without a
specified size, only indices in the declared range are valid.

-Warray-bounds turns on the warnings about out-of-bounds array accesses.
-Warray-bounds=2 does some more warnings, with higher false positive rate. But
it doesn't seem to yield any false positives in our codebase, so enable it.

clang supports -Warray-bounds, but not -Warray-bounds=2.
gcc supports both.
gcc-13 supports -fstrict-flex-arrays.

See https://people.kernel.org/kees/bounded-flexible-arrays-in-c for a long
discussion of use in the kernel.