[fuchsia] Propagate the userspace UTC clock

On Fuchsia, spawning a subprocess does not automatically
clone all of the parent process' capabilities. UTC time on
Fuchsia is managed by a top-level userspace clock capability
that is cloned and passed to subprocesses.

This change ensures that any Rust subprocess gets access to the
UTC clock, if the parent had access to it. This is critical for
tests, which on Fuchsia, use panic=abort and spawn subprocesses
per test.

library/std/src/sys/unix/process/process_fuchsia.rs

@@ -118,7 +118,8 @@ unsafe fn do_exec(
             FDIO_SPAWN_CLONE_JOB
                 | FDIO_SPAWN_CLONE_LDSVC
                 | FDIO_SPAWN_CLONE_NAMESPACE
-                | FDIO_SPAWN_CLONE_ENVIRON, // this is ignored when envp is non-null
+                | FDIO_SPAWN_CLONE_ENVIRON // this is ignored when envp is non-null
+                | FDIO_SPAWN_CLONE_UTC_CLOCK,
             self.get_program().as_ptr(),
             self.get_argv().as_ptr(),
             envp,

library/std/src/sys/unix/process/zircon.rs

@@ -138,6 +138,7 @@ pub fn fdio_spawn_etc(
 pub const FDIO_SPAWN_CLONE_NAMESPACE: u32 = 0x0004;
 pub const FDIO_SPAWN_CLONE_STDIO: u32 = 0x0008;
 pub const FDIO_SPAWN_CLONE_ENVIRON: u32 = 0x0010;
+pub const FDIO_SPAWN_CLONE_UTC_CLOCK: u32 = 0x0020;
 pub const FDIO_SPAWN_CLONE_ALL: u32 = 0xFFFF;
 
 // fdio_spawn_etc actions