[libc][NFC] Add src.__support.OSUtil targets conditionally.

Before this change, they were unconditionally added, irrespective of the
availability of the architecture specific pieces.

libc/src/__support/OSUtil/CMakeLists.txt

@@ -1,4 +1,12 @@
-add_subdirectory(linux)
+if(NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+  return()
+endif()
+
+add_subdirectory(${LIBC_TARGET_OS})
+set(target_os_util libc.src.__support.OSUtil.${LIBC_TARGET_OS}.${LIBC_TARGET_OS}_util)
+if(NOT TARGET ${target_os_util})
+  return()
+endif()
 
 add_header_library(
   osutil
@@ -7,5 +15,5 @@ add_header_library(
     quick_exit.h
     syscall.h
   DEPENDS
-    libc.src.__support.OSUtil.linux.linux_util
+    ${target_os_util}
 )

libc/src/__support/OSUtil/linux/CMakeLists.txt

@@ -1,5 +1,8 @@
-add_subdirectory(aarch64)
-add_subdirectory(x86_64)
+if(NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_ARCHITECTURE})
+  return()
+endif()
+
+add_subdirectory(${LIBC_TARGET_ARCHITECTURE})
 
 add_header_library(
   linux_util
@@ -8,7 +11,6 @@ add_header_library(
     quick_exit.h
     syscall.h
   DEPENDS
-    .aarch64.linux_aarch64_util
-    .x86_64.linux_x86_64_util
+    .${LIBC_TARGET_ARCHITECTURE}.linux_${LIBC_TARGET_ARCHITECTURE}_util
     libc.src.__support.common
 )