adds more (and fixes existing) GCC-specific builtin floating types (#40)

* adds the C99 _Bool type

Note, this type is different from any integer types as, in
```
_Bool a=1, b=2;
assert(a == b);
```
the assertion holds.

* adds more (and fixes existing) GCC-specific builtin floating types

After reading more carefully this document [1] we noticed that some of the
types were missing and some were misnamed

[1]: https://gcc.gnu.org/onlinedocs/gcc/Floating-Types.html

Author: ivg

frontc/clexer.mll

@@ -141,9 +141,18 @@ let keywords =
     ("extern", id EXTERN);
     ("long", id LONG);
     ("_Complex", id COMPLEX);
-    builtin "_Float128";
+    builtin "__float128";
+    builtin "__float80";
+    builtin "__ibm128";
+    builtin "_Float16";
+    builtin "_Float32";
+    builtin "_Float32x";
+    builtin "_Float64";
+    builtin "_Float64x";
     builtin "_Float80";
-    builtin "_Ibm80";
+    builtin "_Float128";
+    builtin "_Float128x";
+    builtin "_Ibm128";
     ("short", id SHORT);
     ("register", id REGISTER);
     ("signed", id SIGNED);