GCC15 tracker

[SchrodingerZhu]

Tracking compatibility with upcoming GCC 15 release.

Known issues:

• [15 Regression] GIMPLE pass asan0 no longer works after r15-5528
• [15 Regression] gcc_jit_block_add_assignment_op GCC_JIT_BINARY_OP_BITWISE_XOR rejects vector types

Planned new features:

• target builtin
• unnamed tmp variable