refactor: use flexible arrays instead of the length-of-one trick (#22072)

The "length-of-one" trick, where the last element of a struct is an
array of size 1, but extra size is allocated when calling malloc where
it uses more than 1 element in the array, cause problems with some
compilers. Some compilers set _FORTIFY_SOURCE=2 by default which
incorrectly considers it as an overflow. More information:
https://github.com/neovim/neovim/issues/223#issuecomment-1413828554

Using flexible array members allows us to to properly convey to the
compiler that its size may be larger than 1. This also enables us to
remove lengthy workarounds that are unreliable, as they depend on
CMAKE_BUILD_TYPE which isn't defined for multi-config generators.

Closes: https://github.com/neovim/neovim/issues/223

src/nvim/spellfile.c

@@ -404,7 +404,7 @@ typedef struct sblock_S sblock_T;
 struct sblock_S {
   int sb_used;                  // nr of bytes already in use
   sblock_T *sb_next;         // next block in list
-  char_u sb_data[1];            // data, actually longer
+  char_u sb_data[];            // data
 };
 
 // A node in the tree.