Implement full GCC-style extended inline assembly for ARM64 backend:
- Add constraint parsing (constraint_priority, skip_constraint_modifiers)
- Implement register allocation (asm_compute_constraints)
- Add code generation for prolog/epilog and load/store (asm_gen_code)
- Support output/input/read-write operands with r, w, f, x, m, g constraints
- Support immediate constraints (i, I, J, K, L, n)
- Handle clobber lists (registers, memory, cc)
- Support constraint references, early clobber, named operands
- Fix '#' character handling in tccpp.c for ARM64 asm mode
Tests: Add comprehensive test suite with 18 test cases covering all features.
All existing TCC tests continue to pass.
