libpayload: Don't sneak in compiler includes

The way we got to include the compiler includes was kind of whacky.
Instead of mixing in potentially problematic headers, make libpayload
self-contained by adding some missing header files. Also clean up
conflicting definitions of size_t throughout the tree.

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

Change-Id: I0ad1194de1a00b7133c5477c00eb167d63a2ee85
Reviewed-on: https://gerrit.chromium.org/gerrit/47608
Reviewed-by: Ronald G. Minnich <rminnich@chromium.org>
Commit-Queue: Stefan Reinauer <reinauer@google.com>
Tested-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/3058
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

payloads/libpayload/include/stdarg.h

@@ -31,7 +31,20 @@
 #define _LIBPAYLOAD_STDARG_H
 
 #include <stddef.h>
+
+/* With GCC we use -nostdinc -ffreestanding to keep out system includes.
+ * Unfortunately this also gets us rid of the _compiler_ includes, like
+ * stdarg.h. To work around the issue, we define varargs directly here.
+ * On LLVM we can still just include stdarg.h.
+ */
+#ifdef __GNUC__
+#define va_start(v,l)		__builtin_va_start(v,l)
+#define va_end(v)		__builtin_va_end(v)
+#define va_arg(v,l)		__builtin_va_arg(v,l)
+typedef __builtin_va_list	va_list;
+#else
 #include_next <stdarg.h>
+#endif
 
 /**
  * @defgroup vprintf Varargs print functions