[libc] Forward CUDA options to the runtimes invocation of `libc`

[jhuber6]

Summary

Some configurations may require -DCUDAToolkit_ROOT to find CUDA
properly. This is currently not forwarded to the CMake invocation. This
patch adds a prefix so it will be visible when the runtimes build is
started.

llvm/runtimes/CMakeLists.txt

foreach(entry ${runtimes})
   # Many compiler-rt options start with SANITIZER_ and DARWIN_ rather than
   # COMPILER_RT_, so when compiler-rt is enabled, consider both.
   if(canon_name STREQUAL "COMPILER_RT")
     list(APPEND prefixes SANITIZER DARWIN)
   endif()
   if(canon_name STREQUAL "LIBC")
     list(APPEND prefixes "LLVM_LIBC")
     list(APPEND prefixes "LIBC_")
+    # The `libc` project may require '-DCUDAToolkit_ROOT' in GPU mode.
+    if(LIBC_GPU_BUILD OR LIBC_GPU_ARCHITECTURES)
+      list(APPEND prefixes "CUDA")
+    endif()
   endif()
 
   string(FIND ${projName} "lib" LIB_IDX)
   if(LIB_IDX EQUAL 0 AND NOT projName STREQUAL "libc")
     string(SUBSTRING ${projName} 3 -1 projName)
   endif()
   list(APPEND runtime_names ${projName})
 endforeach()