[Runtimes] Allow HandleLibc.cmake to be called multiple times (#193540)

Summary:
This needs to check to see if it's already been called now that we want
to use it more places than just libcxx.

runtimes/cmake/Modules/HandleLibC.cmake

@@ -10,6 +10,10 @@
 
 include_guard(GLOBAL)
 
+if(TARGET runtimes-libc-headers)
+  return()
+endif()
+
 set(RUNTIMES_SUPPORTED_C_LIBRARIES system llvm-libc picolibc newlib)
 set(RUNTIMES_USE_LIBC "system" CACHE STRING "Specify C library to use. Supported values are ${RUNTIMES_SUPPORTED_C_LIBRARIES}.")
 if (NOT "${RUNTIMES_USE_LIBC}" IN_LIST RUNTIMES_SUPPORTED_C_LIBRARIES)