[CMake] Specify byproducts for custom libcxx builds

These were removed in bda3f2dd76 but
are needed as it turned out for the MSan tests.

compiler-rt/cmake/Modules/AddCompilerRT.cmake

@@ -654,6 +654,7 @@ macro(add_custom_libcxx name prefix)
     USES_TERMINAL_INSTALL 1
     LIST_SEPARATOR |
     EXCLUDE_FROM_ALL TRUE
+    BUILD_BYPRODUCTS "${prefix}/lib/libc++.a" "${prefix}/lib/libc++abi.a"
     )
 
   if (CMAKE_GENERATOR MATCHES "Make")