random: fix crash on some 64bit platforms

rbx needs to be stashed in a 64bit register on 64bit platforms. With this crash
in particular, it was holding a stack canary which was not properly restored
after the cpuid.

Split out the x86+PIC case so that x86_64 doesn't have to worry about it.

src/random.cpp

@@ -72,10 +72,16 @@ static bool rdrand_supported = false;
 static constexpr uint32_t CPUID_F1_ECX_RDRAND = 0x40000000;
 static void RDRandInit()
 {
-    //! When calling cpuid function #1, ecx register will have this set if RDRAND is available.
+    uint32_t eax, ecx, edx;
+#if defined(__i386__) && ( defined(__PIC__) || defined(__PIE__))
     // Avoid clobbering ebx, as that is used for PIC on x86.
-    uint32_t eax, tmp, ecx, edx;
+    uint32_t tmp;
     __asm__ ("mov %%ebx, %1; cpuid; mov %1, %%ebx": "=a"(eax), "=g"(tmp), "=c"(ecx), "=d"(edx) : "a"(1));
+#else
+    uint32_t ebx;
+    __asm__ ("cpuid": "=a"(eax), "=b"(ebx), "=c"(ecx), "=d"(edx) : "a"(1));
+#endif
+    //! When calling cpuid function #1, ecx register will have this set if RDRAND is available.
     if (ecx & CPUID_F1_ECX_RDRAND) {
         LogPrintf("Using RdRand as entropy source\n");
         rdrand_supported = true;