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 {

Kernel/Arch/aarch64/Processor.cpp

@@ -44,6 +44,8 @@ void Processor::initialize()
     dmesgln("CPU[{}]: Supports {}", m_cpu, build_cpu_feature_names(m_features));
     dmesgln("CPU[{}]: Physical address bit width: {}", m_cpu, m_physical_address_bit_width);
     dmesgln("CPU[{}]: Virtual address bit width: {}", m_cpu, m_virtual_address_bit_width);
+    if (!has_feature(CPUFeature::RNG))
+        dmesgln("CPU[{}]: {} not detected, randomness will be poor", m_cpu, cpu_feature_to_description(CPUFeature::RNG));
 }
 
 [[noreturn]] void Processor::halt()