(bug caused by tcc intristics)

reproduce bug:
$ ./configure --cc=gcc
$ make
$ make install
(OK)
run a test:

extern int printf(const char *str, ...);
int main()
{
        int t2 = (int)(-1.847759065f * 4096);
        printf("%d\n", t2);
}

$ tcc test.c
$ ./a.out
$ -7568
(OK)

(self compiled now)
$ ./configure --cc=tcc
$ make
$ make install
(OK)

$ tcc test.c
$ ./a.out
$ 7568
(WRONG!!!)

why:
gcc does not have intristics for
uint to long double conversion
therefore it does cast implicitly, so
the sign bit is preserved, but this does
not happen when __fixunsxfdi is called
because tcc was bootstrapped.

solution:
force cast to int64 and preserve the
sign bit.

side effects:
not found.

tccgen.c

@@ -3478,7 +3478,7 @@ again:
                 vtop->c.i = (vtop->c.ld != 0);
             } else {
                 if(sf)
-                    vtop->c.i = vtop->c.ld;
+                    vtop->c.i = (int64_t)vtop->c.ld;
                 else if (sbt_bt == VT_LLONG || (PTR_SIZE == 8 && sbt == VT_PTR))
                     ;
                 else if (sbt & VT_UNSIGNED)