[gn] port a41a4ac

Author: nico

llvm/utils/gn/secondary/compiler-rt/lib/builtins/BUILD.gn

@@ -571,6 +571,7 @@ static_library("builtins") {
 
   if (current_cpu == "riscv" || current_cpu == "riscv64") {
     sources += [
+      "riscv/feature_bits.c",
       "riscv/fp_mode.c",
       "riscv/restore.S",
       "riscv/save.S",