tccgen: fix long long -> char/short cast

This was causing assembler bugs in a tcc compiled by itself
at i386-asm.c:352 when ExprValue.v was changed to uint64_t:

    if (op->e.v == (int8_t)op->e.v)
        op->type |= OP_IM8S;

A general test case:

    #include <stdio.h>
    int main(int argc, char **argv)
    {
        long long ll = 4000;
        int i = (char)ll;
        printf("%d\n", i);
        return 0;
    }

Output was "4000", now "-96".

Also: add "asmtest2" as asmtest with tcc compiled by itself

tests/asmtest.S

@@ -696,6 +696,7 @@ int $0x10
     inc %eax
 #endif
 
+#ifndef _WIN32
 ft1: ft2: ft3: ft4: ft5: ft6: ft7: ft8: ft9:
     xor %eax, %eax
     ret
@@ -708,6 +709,7 @@ ft1: ft2: ft3: ft4: ft5: ft6: ft7: ft8: ft9:
 .type ft6,@function
 .type ft7,%function
 .type ft8,"function"
+#endif
 
     pause
 .rept 6