Merge upstream GCC

Any chance upstream GCC may be merged?  Latest version (15.x) includes some RISC-V-specific enhancements such as [support for aligned register pairs](https://github.com/gcc-mirror/gcc/commit/fcbb8456a58ba073d4d5b10fcb9057b6e9a100db), as well as built-in support for some CORE-V extensions such as Xcvmac and Xcvsimd (but not Xcvhwlp, for instance).