[libc][malloc] Ensure a minimum block alignment of 4

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.

Author: mysterymath

libc/src/__support/block.h

@@ -90,7 +90,10 @@ using cpp::optional;
 ///
 /// The next offset of a block matches the previous offset of its next block.
 /// The first block in a list is denoted by having a previous offset of `0`.
-class Block {
+///
+// Ensure a minimum alignment of 4, since at least 2 bits must be available in
+// block sizes for flags.
+class alignas(cpp::max(alignof(size_t), size_t{4})) Block {
   // Masks for the contents of the next_ field.
   static constexpr size_t PREV_FREE_MASK = 1 << 0;
   static constexpr size_t LAST_MASK = 1 << 1;
@@ -360,9 +363,6 @@ class Block {
   static constexpr size_t PREV_FIELD_SIZE = sizeof(prev_);
 };
 
-static_assert(alignof(Block) >= 4,
-              "at least 2 bits must be available in block sizes for flags");
-
 LIBC_INLINE
 optional<Block *> Block::init(ByteSpan region) {
   if (!region.data())