Add relibc fenv and sched POSIX implementations

P3-fenv: x86_64 SSE/x87 inline asm for 10 FP environment functions
(feclearexcept, fegetenv, fegetexceptflag, fegetround, feholdexcept,
feraiseexcept, fesetenv, fesetexceptflag, fesetround, fetestexcept,
feupdateenv) with proper MXCSR and x87 CW register access.

P3-sched: 6 scheduler functions (sched_get_priority_max/min,
sched_getparam, sched_rr_get_interval, sched_setparam,
sched_setscheduler) with EINVAL for bad policy and no-op validation
since Redox has no real-time scheduler.

recipes/core/relibc/recipe.toml

@@ -34,6 +34,8 @@ patches = [
     "../../../local/patches/relibc/P3-sysv-sem-impl.patch",
     "../../../local/patches/relibc/P3-sysv-shm-impl.patch",
     "../../../local/patches/relibc/P3-waitid-header.patch",
+    "../../../local/patches/relibc/P3-fenv.patch",
+    "../../../local/patches/relibc/P3-sched.patch",
 ]
 
 [build]