Follow rename of mx_handle_wait Magenta syscalls

The mx_handle_wait_* syscalls in Magenta were renamed to
mx_object_wait. The syscall is used in the Magenta/Fuchsia
implementation of std::process, to wait on child processes.

In addition, this patch enables the use of the system provided
libbacktrace library on Fuchsia targets. Symbolization is not yet
working, but at least it allows printing hex addresses in a backtrace
and makes building succeed when the backtrace feature is not disabled.

Author: raphlinus

src/libstd/build.rs

@@ -59,6 +59,10 @@ fn main() {
         println!("cargo:rustc-link-lib=userenv");
         println!("cargo:rustc-link-lib=shell32");
     } else if target.contains("fuchsia") {
+        // use system-provided libbacktrace
+        if cfg!(feature = "backtrace") {
+            println!("cargo:rustc-link-lib=backtrace");
+        }
         println!("cargo:rustc-link-lib=magenta");
         println!("cargo:rustc-link-lib=mxio");
         println!("cargo:rustc-link-lib=launchpad"); // for std::process

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

@@ -111,7 +111,7 @@ extern {
     pub fn mx_handle_duplicate(handle: mx_handle_t, rights: mx_rights_t,
                                out: *const mx_handle_t) -> mx_handle_t;
 
-    pub fn mx_handle_wait_one(handle: mx_handle_t, signals: mx_signals_t, timeout: mx_time_t,
+    pub fn mx_object_wait_one(handle: mx_handle_t, signals: mx_signals_t, timeout: mx_time_t,
                               pending: *mut mx_signals_t) -> mx_status_t;
 
     pub fn mx_object_get_info(handle: mx_handle_t, topic: u32, buffer: *mut c_void,

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

@@ -151,7 +151,7 @@ impl Process {
         let mut avail: mx_size_t = 0;
 
         unsafe {
-            mx_cvt(mx_handle_wait_one(self.handle.raw(), MX_TASK_TERMINATED,
+            mx_cvt(mx_object_wait_one(self.handle.raw(), MX_TASK_TERMINATED,
                                       MX_TIME_INFINITE, ptr::null_mut()))?;
             mx_cvt(mx_object_get_info(self.handle.raw(), MX_INFO_PROCESS,
                                       &mut proc_info as *mut _ as *mut libc::c_void,
@@ -174,7 +174,7 @@ impl Process {
         let mut avail: mx_size_t = 0;
 
         unsafe {
-            let status = mx_handle_wait_one(self.handle.raw(), MX_TASK_TERMINATED,
+            let status = mx_object_wait_one(self.handle.raw(), MX_TASK_TERMINATED,
                                             0, ptr::null_mut());
             match status {
                 0 => { }, // Success