alignas (C11)

Here is another patch set to sprinkle some C11 features around the
code.  My aim is to make a little bit of use of several C11 features
as examples and encouragement for future code, and to test compilers.

Here, I'm proposing to make some use of the alignas specifier.  This 
takes the place of compiler extensions such as 
__attribute__((aligned(a))) and __declspec(align(a)), packaged up as 
pg_attribute_aligned(a), which are used in a variety of places.  Also, 
we can simplify some places where unions are used to encourage 
alignment, and remove a few workaround for lack of alignment attribute 
support.

Some detail notes:

- Technically, compilers are only required to support alignas up to 
(handwaving over some terminology) the largest alignment of a built-in 
type, so maybe 8 or 16.  Support for larger alignments like 
alignas(PG_CACHE_LINE_SIZE) is implementation-defined.  I have split up 
my patches so that fundamental and extended alignments are in separate 
patches, so this could be eased into, but I'm expecting that all 
compilers in practical use support alignments up to PG_IO_ALIGN_SIZE. 
(For MSVC, 4096 appears to be the actual limit by default, per [0], but 
this is independent of using alignas or __declspec.  I haven't found any 
explicit documentation for clang or gcc.)

[0]: 
https://learn.microsoft.com/en-us/cpp/build/reference/align-section-alignment?view=msvc-170

- You cannot use alignas on a typedef.  So some uses of the attribute 
pg_attribute_aligned() like for PgAioUringContext or the whole int128 
business cannot be converted directly.  The solution for cases like 
PgAioUringContext could be to move the alignas into the struct, but I 
haven't studied this code closely enough, so I'm leaving it.  For 
int128, there is no straightforward solution, so I'm also leaving that 
as is.

(The reason for this restriction is that typedefs are supposed to be 
type aliases that are interchangeable.  But if you have two otherwise 
compatible typedefs with different alignments, this kind of violates the 
C type system and the compiler has to do some nonstandard magic to 
handle this (or fail to, see "checking for __int128 alignment bug").)

- You cannot use alignas to underalign a type.  So again, int128 cannot 
be handled by this.

For at least these reasons, I'm leaving pg_attribute_aligned() and some 
of its more tricky uses in place and unchanged.