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.
1 job for master in 70 minutes and 42 seconds (queued for 4 seconds)
Status Job ID Name Coverage
failed #1541


Name Stage Failure
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:315: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check-expensive] Error 2
Makefile:290: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:962: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1