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.