Builtins patch causes recompilation if 1k+ files

This annoying as the compilation time after adding intrinsics is 5x longer than without.

I guess the reason for this behavior is that all builtins patches for all targets are processed by a single tablegen backend. While on the LLVM side only the RISC-V specific files need to be re-generated, there is no way to rebuild only the RISC-V related Clang sources.