Improve compound literal support

This commit consolidates compound literal improvements:
- Support global struct/array initialization with compound literals
- Implement GP-relative stores for global array initializers
- Enhanced global struct variable declaration support
- Support nested compound literals in arrays

What still doesn't work:
- True C99 compound literals with cast syntax: (Type){...}
- Struct compound literals: (struct Point){10, 20}
- Array compound literals with type: (int[]){1, 2, 3}

Author: jserv