Minor tweaks to recent UBSan-related fix
* src/alloc.c: No need to include stdalign.h; it’s pervasive. (GC_STRING_OVERRUN_COOKIE_SIZE): Align to sdata’s alignment, so that the code works even if alignof (sdata) exceeds 8. Don’t require the cookie size to be 8, as this overly fattens 32-bit platforms and one DEADBEEF should be enough. (GC_STRING_EXTRA): Omit now-unnecessary ‘verify’. (allocate_string_data): Omit unnecessary cast.
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