[CMake][runtimes] Don't passthrough prefixes for non-default targets

[phosek]

Summary

The passthrough is useful for setting up the options for the default
build, but we already have a different mechanism to pass CMake flags
to builds for builtins and runtimes targets so this is not really
needed there. Furthermore, when the flags are set for the default
build, with the prefix passthrough set we have to explicitly override
all options in other targets which can be cumbersome.

[phosek]

ping

[beanz]

LGTM

llvm/trunk/runtimes/CMakeLists.txt

llvm_ExternalProject_Add(builtins-${target}
                                         -DLLVM_RUNTIME_OUTPUT_INTDIR=${LLVM_TOOLS_BINARY_DIR}
                                         -DCMAKE_C_COMPILER_TARGET=${target}
                                         -DCMAKE_ASM_COMPILER_TARGET=${target}
                                         -DCMAKE_C_COMPILER_WORKS=ON
                                         -DCMAKE_ASM_COMPILER_WORKS=ON
                                         -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON
                                         ${${target}_extra_args}
                              TOOLCHAIN_TOOLS clang lld llvm-ar llvm-ranlib
-                             PASSTHROUGH_PREFIXES COMPILER_RT
                              USE_TOOLCHAIN
                              ${EXTRA_ARGS})
   endfunction()
 
   # If compiler-rt is present we need to build the builtin libraries first. This
   # is required because the other runtimes need the builtin libraries present
   # before the just-built compiler can pass the configuration tests.
   if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/compiler-rt)