Insert spaces between certain tokens when tcc is invoked with -E.

Insert a space when it is required to prevent mistokenisation of
the output, and also in a few cases where it is not strictly
required, imitating GCC's behaviour.

tests/pp/15.c

@@ -19,3 +19,8 @@ return A + B;
 #define B1 C1+2
 #define C1 A1+3
 return A1 + B1;
+
+#define i() x
+#define n() 1
+i()i()n()n()i()
+i()+i()-n()+n()-