Backport: fix broken build on m68k

Backport: fix broken build on m68k

The GCC + valgrind fix caused the m68k build to fail (Bug#35711).
Simplify string allocation a bit to make similar problems less
likely in the future.
* src/alloc.c (sdata, SDATA_NBYTES, SDATA_DATA) [GC_CHECK_STRING_BYTES]:
Use the same implementation as with !GC_CHECK_STRING_BYTES,
as the special case is no longer needed.
(SDATA_ALIGN): New constant.
(SDATA_SIZE): Remove this macro, replacing with ...
(sdata_size): ... this new function.  All uses changed.
Properly account for sizes and alignments even in the m68k case,
and even if GC_CHECK_STRING_BYTES is not defined.