[cmake] Normalize TARGET_SUBDIR paths (#138524)

Some code paths normalize ".." and thus don't create the directory. But some execute in a
shell thus requiring the directory to exist to be able to take the parent directory.

This patch normalizes all the `TARGET_SUBDIR` variables to avoid this issue.

GitOrigin-RevId: fc3b67aee852d99bbb5d2f4670695d8246375dac
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6dcfc51..3e2f80b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -187,6 +187,7 @@
   if(LIBCXXABI_LIBDIR_SUBDIR)
     string(APPEND LIBCXXABI_TARGET_SUBDIR /${LIBCXXABI_LIBDIR_SUBDIR})
   endif()
+  cmake_path(NORMAL_PATH LIBCXXABI_TARGET_SUBDIR)
   set(LIBCXXABI_HEADER_DIR ${LLVM_BINARY_DIR})
   set(LIBCXXABI_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LIBCXXABI_TARGET_SUBDIR})
   set(LIBCXXABI_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LIBCXXABI_TARGET_SUBDIR} CACHE STRING