[libc][NFC][Obvious] Tidy up some CMake files.

Conditionally adding subdirectories was missed in a few places previously.
This change adds the conditionals. A sub-directory was being added
needlessly in another place. That has been removed.

GitOrigin-RevId: 235e9cc9243109079d68ca1e387069606b517d44
diff --git a/config/linux/CMakeLists.txt b/config/linux/CMakeLists.txt
index 6fdfb8c..c8f496b 100644
--- a/config/linux/CMakeLists.txt
+++ b/config/linux/CMakeLists.txt
@@ -9,5 +9,3 @@
   DEPENDS
     libc.src.__support.common
 )
-
-add_subdirectory(x86_64)
diff --git a/config/linux/x86_64/CMakeLists.txt b/config/linux/x86_64/CMakeLists.txt
deleted file mode 100644
index e69de29..0000000
--- a/config/linux/x86_64/CMakeLists.txt
+++ /dev/null
diff --git a/src/string/CMakeLists.txt b/src/string/CMakeLists.txt
index 634e5f9..114fce7 100644
--- a/src/string/CMakeLists.txt
+++ b/src/string/CMakeLists.txt
@@ -132,4 +132,6 @@
 # Add all other relevant implementations for the native target.
 # ------------------------------------------------------------------------------
 
-include(${LIBC_STRING_TARGET_ARCH}/CMakeLists.txt)
+if(EXISTS ${LIBC_STRING_TARGET_ARCH})
+  include(${LIBC_STRING_TARGET_ARCH}/CMakeLists.txt)
+endif()
diff --git a/test/config/linux/CMakeLists.txt b/test/config/linux/CMakeLists.txt
index 7dc887c..caff2ef 100644
--- a/test/config/linux/CMakeLists.txt
+++ b/test/config/linux/CMakeLists.txt
@@ -1,3 +1,5 @@
 add_libc_testsuite(libc_linux_tests)
 
-add_subdirectory(${LIBC_TARGET_MACHINE})
+if(EXISTS ${LIBC_TARGET_MACHINE})
+  add_subdirectory(${LIBC_TARGET_MACHINE})
+endif()