Auto merge of #42452 - raphlinus:mx_job_default, r=alexcrichton

[fuchsia] Track change of mx_job_default

The implementation of mx_job_default changed from a macro which
accessed the __magenta_job_default global variable to a proper
function call. This patch tracks that change.

src/libstd/sys/unix/process/magenta.rs

@@ -102,7 +102,7 @@ pub struct mx_info_process_t {
 }
 
 extern {
-    static __magenta_job_default: mx_handle_t;
+    pub fn mx_job_default() -> mx_handle_t;
 
     pub fn mx_task_kill(handle: mx_handle_t) -> mx_status_t;
 
@@ -119,10 +119,6 @@ extern {
                               avail: *mut mx_size_t) -> mx_status_t;
 }
 
-pub fn mx_job_default() -> mx_handle_t {
-    unsafe { return __magenta_job_default; }
-}
-
 // From `enum special_handles` in system/ulib/launchpad/launchpad.c
 // HND_LOADER_SVC = 0
 // HND_EXEC_VMO = 1