Aligning blocks to max_align_t is neither necessary nor sufficient to
ensure that the usable_space() is so aligned. Instead, we make this an
invariant of Block and maintain it in init() and split().

This allows targets like riscv32 with small pointers and large
max_align_t to maintain the property that all available blocks are
aligned for malloc(). This change adjusts the tests to match and also
updates them closer to llvm-libc style.

This reverts commit 93b83642ee34d0092b94776728dad0117c2b72a1.

 - Correct riscv32 assumption about alignment (bit of a hack).
 - Fix test case where the largest_small and smallest sizes are the
   same.

This reverts commit b05600d96f46697e21f6b1b7ad901391326243a8.
riscv32 unit test still broken

 - Fix assertion expressions.
 - Fix incorrect small size in freestore_test.
 - There may only be one small size for high alignment and small
   pointers (riscv32).
 - Don't rely on stack alignment in FreeList test.

Reverts llvm/llvm-project#106259

Unit tests break on AArch64.

This reworks the free store implementation in libc's malloc to use a
dlmalloc-style binary trie of circularly linked FIFO free lists. This
data structure can be maintained in logarithmic time, but it still
permits a relatively small implementation compared to other
logarithmic-time ordered maps.

The implementation doesn't do the various bitwise tricks or
optimizations used in actual dlmalloc; it instead optimizes for
(relative) readability and minimum code size. Specific optimization can
be added as necessary given future profiling.