[CMake] Change GCC_INSTALL_PREFIX from warning to fatal error (#85891)

unless USE_DEPRECATED_GCC_INSTALL_PREFIX (temporary escape hatch) is
set. Setting GCC_INSTALL_PREFIX leads to a warning for Clang 18.1
(#77537) and will be completely removed for Clang 20.

Link:
discourse.llvm.org/t/add-gcc-install-dir-deprecate-gcc-toolchain-and-remove-gcc-install-prefix/65091
Link:
discourse.llvm.org/t/correct-cmake-parameters-for-building-clang-and-lld-for-riscv/72833