If I understand correctly, how about `__alignof__', utilizing GCC's
knowledge of the architecture?
$ cat a.c
#include <stdio.h>
#ifdef __GNUC__
size_t alignment = __alignof__ (double);
#else
size_t alignment = 8;
#endif
main () { printf ("%d\n", alignment); exit (0); }
$ arch && cc a.c && ./a.out
i586
4
---
$ arch && cc a.c && ./a.out
sparc64
8