[libc][stdlib] Add Block class (#94407)

A block represents a chunk of memory used by the freelist allocator. It
contains header information denoting the usable space and pointers as
offsets to the next and previous block.

On it's own, this doesn't do much. This is a part of
https://github.com/llvm/llvm-project/pull/94270 to land in smaller
patches.

This is a subset of pigweed's freelist allocator implementation.