[compiler-rt][RISCV] Use muldi3 builtin assembly implementation

D80465 added an assembly implementation of muldi3 for RISC-V but it didn't
add it to the cmake `*_SOURCES` list, so the C implementation was being used
instead. This patch fixes that.

Differential Revision: https://reviews.llvm.org/D86036

compiler-rt/lib/builtins/CMakeLists.txt

@@ -574,7 +574,10 @@ set(riscv32_SOURCES
   riscv/mulsi3.S
   ${riscv_SOURCES}
 )
-set(riscv64_SOURCES ${riscv_SOURCES})
+set(riscv64_SOURCES
+  riscv/muldi3.S
+  ${riscv_SOURCES}
+)
 
 set(sparc_SOURCES ${GENERIC_SOURCES} ${GENERIC_TF_SOURCES})
 set(sparcv9_SOURCES ${GENERIC_SOURCES} ${GENERIC_TF_SOURCES})