redox_syscall: add AcpiVerb::SetLpiHint (verb 7) for LPIT MWAIT hint

acpid provides the firmware-recommended MWAIT hint from the LPIT
native C-state entry_trigger (FFH GAS address). The kernel idle path
uses this hint for mwait_loop() instead of the CPUID-max fallback.
Write payload: 4-byte LE u32. Read payload: 0 (one-way).

src/flag.rs

@@ -355,6 +355,13 @@ pub enum AcpiVerb {
     /// Mirrors Linux 7.1's `enter_sleep_state` /
     /// `acpi_hw_legacy_sleep` for S3.
     EnterS3 = 6,
+    /// Red Bear OS extension (Phase I): acpid provides the
+    /// firmware-recommended MWAIT hint from the LPIT native
+    /// C-state entry_trigger (FFH GAS address). The kernel idle
+    /// path uses this hint for `mwait_loop()` instead of the
+    /// CPUID-max fallback. Write payload: 4-byte LE u32 (the
+    /// hint). Read payload: 0 (one-way).
+    SetLpiHint = 7,
 }
 impl AcpiVerb {
     pub const fn try_from_raw(value: u64) -> Option<Self> {
@@ -365,6 +372,7 @@ impl AcpiVerb {
             4 => Self::ExitS2Idle,
             5 => Self::SetS3WakingVector,
             6 => Self::EnterS3,
+            7 => Self::SetLpiHint,
             _ => return None,
         })
     }