Fix alignment error

[alessandropellegrini]

This is a quick&dirty fix for an alignment error in the buddy system. Fixes #105.

Per the C standard, malloc() returns memory aligned to max_align_t, typically 8 bytes on some LP64 platforms (e.g., many Linux/ARM64 configurations) and 16 bytes on x86-64. The C standard does not guarantee
malloc() satisfies over-aligned types (with
alignment greater than max_align_t).

Possible problems in the previous implementation:

• Undefined behavior: Accessing a misaligned alignas(16) member is UB per C11 §6.2.8.
• SIMD crashes: If the compiler auto-vectorizes operations on longest[] or base_mem[] and emits SSE aligned loads/stores (movaps, ld1), a misaligned buffer will cause a hardware fault.
• Checkpoint corruption: The static_assert in buddy.h enforces contiguity between longest and base_mem. Misalignment of the struct could interact poorly with memcpy-based checkpoint/restore paths (checkpoint.c) if the compiler assumes alignment it doesn't actually have. I wasn't actually able to trigger this latter condition in any environment, but it should be nevertheless possible from my understanding.

On x86-64, malloc() typically returns 16-byte-aligned memory, so this bug is latent on the most common targets. It becomes active on platforms where max_align_t alignment is less than 16.
Since we pretend to be architecture-independent, I classify this as a severe bug.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.32%. Comparing base (15c55e2) to head (cdd3b5e).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #126      +/-   ##
===========================================
- Coverage    85.81%   85.32%   -0.49%     
===========================================
  Files           46       46              
  Lines         1600     1554      -46     
  Branches        39       39              
===========================================
- Hits          1373     1326      -47     
- Misses         211      212       +1     
  Partials        16       16              

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[Piccions]

Quick and dirty does the job here!