Add #define REGc

Originally committed as revision 5988 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/x86_cpu.h

@@ -15,6 +15,7 @@
 #  define REGBP   rbp
 #  define REGa    rax
 #  define REGb    rbx
+#  define REGc    rcx
 #  define REGSP   rsp
 
 #else
@@ -32,6 +33,7 @@
 #  define REGBP   ebp
 #  define REGa    eax
 #  define REGb    ebx
+#  define REGc    ecx
 #  define REGSP   esp
 #endif