Add support of x86-64.

Most change was done in #ifdef TCC_TARGET_X86_64. So, nothing should be broken by this change.

Summary of current status of x86-64 support:

- produces x86-64 object files and executables.
- the x86-64 code generator is based on x86's.
-- for long long integers, we use 64bit registers instead of tcc's generic implementation.
-- for float or double, we use SSE. SSE registers are not utilized well (we only use xmm0 and xmm1).
-- for long double, we use x87 FPU.
- passes make test.
- passes ./libtcc_test.
- can compile tcc.c. The compiled tcc can compile tcc.c, too. (there should be some bugs since the binary size of tcc2 and tcc3 is differ where tcc tcc.c -o tcc2 and tcc2 tcc.c -o tcc3)
- can compile links browser. It seems working.
- not tested well. I tested this work only on my linux box with few programs.
- calling convention of long-double-integer or struct is not exactly the same as GCC's x86-64 ABI.
- implementation of tcc -run is naive (tcc -run tcctest.c works, but tcc -run tcc.c doesn't work). Relocating 64bit addresses seems to be not as simple as 32bit environments.
- shared object support isn't unimplemented
- no bounds checker support
- some builtin functions such as __divdi3 aren't supported

configure

@@ -39,6 +39,9 @@ case "$cpu" in
   i386|i486|i586|i686|i86pc|BePC)
     cpu="x86"
   ;;
+  x86_64)
+    cpu="x86-64"
+  ;;
   armv4l)
     cpu="armv4l"
   ;;
@@ -313,6 +316,9 @@ echo "EXESUF=$EXESUF" >> config.mak
 if test "$cpu" = "x86" ; then
   echo "ARCH=i386" >> config.mak
   echo "#define HOST_I386 1" >> $TMPH
+elif test "$cpu" = "x86-64" ; then
+  echo "ARCH=x86-64" >> config.mak
+  echo "#define HOST_X86_64 1" >> $TMPH
 elif test "$cpu" = "armv4l" ; then
   echo "ARCH=arm" >> config.mak
   echo "#define HOST_ARM 1" >> $TMPH