Unified alignment macros

[shirok]

Currently we rely on GCC's __attribute__((aligned ...)) feature for some internal types that requires certain alignments. Macros such as SCM_ALIGN8 and SCM_ALIGN_PAIR hides the details (in gauche.h).

C11 has _Alignas so we want to use it for portability. However, it can only be used for variables or struct members but not in a part of typedef, so we can't simply use it in our macros, which is used for both variable definitions and typedefs.

We can either:

• Replace occurrence of SCM_ALIGN... macros to ensure they are always used for variable/member definitions
• Add a separate macro to be used for varaible/member definitions.