[libc] Fix missing default value for errno config

Summary:
The configs all need default values which targets then override. This
one was an empty string which made the logic report an error. The only
reason it wasn't a build failure was because of a stray `:`.

Author: jhuber6

libc/cmake/modules/LibcConfig.cmake

@@ -113,7 +113,7 @@ function(load_libc_config config_file)
       message(FATAL_ERROR ${json_error})
     endif()
     if(NOT DEFINED ${opt_name})
-      message(FATAL_ERROR: " Option ${opt_name} defined in ${config_file} is invalid.")
+      message(FATAL_ERROR " Option ${opt_name} defined in ${config_file} is invalid.")
     endif()
     if(ARGN)
       list(FIND ARGN ${opt_name} optname_exists)

libc/config/config.json

@@ -1,7 +1,7 @@
 {
   "errno": {
     "LIBC_CONF_ERRNO_MODE": {
-      "value": "",
+      "value": "LIBC_ERRNO_MODE_THREAD_LOCAL",
       "doc": "The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, and LIBC_ERRNO_MODE_SYSTEM."
     }
   },