[libc] Make all targets respect LLVM_PER_TARGET_RUNTIME_DIR (llvm#160597)

Summary:
The libcxx and compiler-rt already install their headers according
to the triple if this option is enabled. We should do this by default so
these don't get mixed up when people potentially combine multiple
toolchains.

Author: jhuber6

libc/CMakeLists.txt

@@ -246,7 +246,7 @@ else()
     set(LIBC_INCLUDE_DIR ${CMAKE_BINARY_DIR}/include)
     set(LIBC_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
   endif()
-  if(LIBC_TARGET_OS_IS_GPU)
+  if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)
     if(LIBC_TARGET_TRIPLE)
       set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/${LIBC_TARGET_TRIPLE})
     else()