fpc_cpuinit: add destroyed register lists to assembler blocks

Otherwise the compiler assumes no registers are overwritten. And while the
regular code generator won't use register variables if assembler blocks are
present, LLVM is not restricted like that (and it could still cause issues
even with the default code generator in case PIC-rebased addresses are
accessed).

rtl/x86_64/x86_64.inc

@@ -1636,7 +1636,7 @@ procedure fpc_cpuinit;
       xorl %ecx,%ecx
       cpuid
       movl %ecx,cpuid1_ecx
-    end;
+    end ['eax', 'ebx', 'ecx', 'edx'];
     has_sse41_support:=boolean(cpuid1_ecx shr 19 and 1);
     if _eax>=7 then
       begin
@@ -1645,7 +1645,7 @@ procedure fpc_cpuinit;
           xorl %ecx,%ecx
           cpuid
           movl %ebx,cpuid7_ebx
-        end;
+        end ['eax', 'ebx', 'ecx', 'edx'];
 {$ifdef use_fast_repmovstos}
         fast_large_repmovstosb:=cpuid7_ebx and (1 shl 9)<>0;
 {$endif}
@@ -1656,7 +1656,7 @@ procedure fpc_cpuinit;
               xorl %ecx,%ecx
               .byte   0x0f,0x01,0xd0 { xgetbv }
               movl %eax,_eax
-            end;
+            end ['eax', 'rcx', 'edx'];
             if (_eax and 6)=6 then
               begin
                 has_avx_support:=(cpuid1_ecx and $10000000)<>0;