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/unistd.h

@@ -32,8 +32,6 @@
 
 #include <stddef.h>
 
-typedef ptrdiff_t ssize_t;
-
 int getpagesize(void);
 
 #endif