[libc][malloc] Ensure a minimum block alignment of 4 (#169447)

Most platforms inherently have a size_t alignment of 4, but this isn't
true on every platform LLVM has some degree of backend support for.
Accordingly, it's simple enough to just set the min alignment of Block
to 4 and lose the static_assert.

NOKEYCHECK=True
GitOrigin-RevId: b76300acc5207b77ffce5677f31491ee58f06c06
7 files changed