tccpp.c: Configurable integer literal overflow handling

Add the define TCC_CUT_ON_INTEGER_LITERAL_OVERFLOW to use the most
significant digits of an integer literal before its parsing led to an
overflow.

When compiling tcc with another compiler, which is not able to handle
64 bit arithmetic, it is beneficial to use the last value before an
integer literal overflows. Parsing 0x1000000000000000 then results in
0x10000000. The mescc from GNU Mes is able to compile a first tcc on a
32 bit system without 64 bit arithmetic. This change allows this first
tcc to compile a second tcc with complete 64 bit arithmetic.

Author: Stefan

tccpp.c

@@ -2550,7 +2550,7 @@ static void parse_number(const char *p)
             }
         }
     } else {
-        unsigned long long n, n1;
+        unsigned long long n = 0, n1 = 0;
         int lcount, ucount, ov = 0;
         const char *p1;
 
@@ -2561,7 +2561,6 @@ static void parse_number(const char *p)
             b = 8;
             q++;
         }
-        n = 0;
         while(1) {
             t = *q++;
             /* no need for checks except for base 10 / 8 errors */
@@ -2575,13 +2574,22 @@ static void parse_number(const char *p)
                 t = t - '0';
             if (t >= b)
                 tcc_error("invalid digit");
-            n1 = n;
             n = n * b + t;
-            /* detect overflow */
-            if (n1 >= 0x1000000000000000ULL && n / b != n1)
-                ov = 1;
+            if (!ov)
+                /* detect overflow */
+                if (n1 >= 0x1000000000000000ULL && n / b != n1)
+                    ov = 1;
+                else
+                    n1 = n;
         }
-
+#ifdef TCC_CUT_ON_INTEGER_LITERAL_OVERFLOW
+        /* On integer literal overflow use the most significant digits before
+           the overflow happened. Effectively this cuts the 0x1000000000000000
+           from above down to 0x10000000 and allows to bootstrap tcc with 32 bit
+           arithmetic. */
+        if (ov)
+            n = n1;
+#endif
         /* Determine the characteristics (unsigned and/or 64bit) the type of
            the constant must have according to the constant suffix(es) */
         lcount = ucount = 0;