Make ssize_t an actual ssize_t

In the process of getting rid of compiler includes during in coreboot
and libpayload, we defined size_t and ssize_t ourselves, using a GCC
macro for size_t: __SIZE_TYPE__. Unfortunately, there is no
__SSIZE_TYPE__, so we temporarily redefine unsigned to signed to make
__SIZE_TYPE__ __SSIZE_TYPE__.

Signed-off-by: Stefan Reinauer <reinauer@google.com>

Change-Id: I4cf4eb0fdaa4db64277c2585fe2c1bdc0acdf02b
Reviewed-on: https://gerrit.chromium.org/gerrit/49947
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Commit-Queue: Stefan Reinauer <reinauer@google.com>
Tested-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/4156
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/include/stddef.h

@@ -6,7 +6,13 @@ typedef long ptrdiff_t;
 #define __SIZE_TYPE__ unsigned long
 #endif
 typedef __SIZE_TYPE__ size_t;
-typedef long ssize_t;
+/* There is a GCC macro for a size_t type, but not
+ * for a ssize_t type. Below construct tricks GCC
+ * into making __SIZE_TYPE__ signed.
+ */
+#define unsigned signed
+typedef __SIZE_TYPE__ ssize_t;
+#undef unsigned
 
 typedef int wchar_t;
 typedef unsigned int wint_t;