Kernel: Use RDSEED assembly snippet to seed RNG on Aarch64

There’s similar RDRAND register (encoded as ‘s3_3_c2_c4_0ʼ) to be
added if needed. RNG CPU feature on Aarch64 guarantees existence of both
RDSEED and RDRAND registers simultaneously—in contrast to x86-64, where
respective instructions are independent of each other.

Kernel/Arch/aarch64/ASM_wrapper.h

@@ -91,6 +91,19 @@ inline void enter_el1_from_el2()
                      : "x0");
 }
 
+inline u64 read_rndrrs()
+{
+    u64 value = 0;
+
+    asm volatile(
+        "retry:\n"
+        "mrs %[value], s3_3_c2_c4_1 \n" // encoded RNDRRS register
+        "b.eq retry\n"
+        : [value] "=r"(value));
+
+    return value;
+}
+
 }
 
 namespace Kernel {