Resolve optional libc entries at runtime, not boot load

A literal (foreign-procedure "chmod" ...) in compiled code becomes a fasl
relocation resolved when the boot loads — on Windows (no chmod/sigemptyset/
sigaddset in the CRT) that killed joltc.exe before any guard could run
(msvcrt abort, exit 3). jolt-foreign-proc-safe defers the lookup through
eval at evaluation time, where the guard works and a missing entry just
yields the fallback. chmod also skips the /bin/sh fallback on nt (execute
is by extension).

host/chez/java/host-static-methods.ss

@@ -55,9 +55,7 @@
     (lambda ()
       (unless tried?
         (set! tried? #t)
-        (set! fp (guard (e (#t #f))
-                   (load-shared-object #f)
-                   (foreign-procedure "sched_yield" () int))))
+        (set! fp (jolt-foreign-proc-safe "sched_yield" '() 'int)))
       (if fp (fp) (sleep (make-time 'time-duration 0 0)))
       jolt-nil)))