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

libtcc1.c

@@ -106,6 +106,9 @@ union float_long {
     long l;
 };
 
+/* XXX: we don't support several builtin supports for now */
+#ifndef __x86_64__
+
 /* XXX: use gcc/tcc intrinsic ? */
 #if defined(__i386__)
 #define sub_ddmmss(sh, sl, ah, al, bh, bl) \
@@ -482,6 +485,8 @@ unsigned short __tcc_fpu_control = 0x137f;
 unsigned short __tcc_int_fpu_control = 0x137f | 0x0c00;
 #endif
 
+#endif /* !__x86_64__ */
+
 /* XXX: fix tcc's code generator to do this instead */
 float __floatundisf(unsigned long long a)
 {