Remove the FP area initialization code

When using the optimized string lib in Occlum, the memset function would
use xmm0 register, as the result, the FP area initialization code would
modify the FP area before saving it. So just ignor the FP area
initialization code.

src/libos/src/syscall/mod.rs

@@ -908,11 +908,11 @@ pub struct FpRegs {
 impl FpRegs {
     /// Save the current CPU floating pointer states to an instance of FpRegs
     pub fn save() -> Self {
-        let mut fpregs = FpRegs {
-            inner: Aligned([0u8; 512]),
-        };
-        unsafe { _fxsave(fpregs.inner.as_mut_ptr()) };
-        fpregs
+        let mut fpregs = MaybeUninit::<Self>::uninit();
+        unsafe {
+            _fxsave(fpregs.as_mut_ptr() as *mut u8);
+            fpregs.assume_init()
+        }
     }
 
     /// Restore the current CPU floating pointer states from this FpRegs instance