Fix libpayload alloc() size and gcc pointer optimization problems.

Fix libpayload alloc() size and gcc pointer optimization problems.

The previous commit was incomplete and missed setting the entire
alloc area.

There are also additional problems with gcc optimizations of the
pointer math. The "auto" casting by gcc wouldn't return warnings,
but it was causing the optimization to be incorrect. We are now
very explicit in the casting in the pointer math.

Change-Id: I020808c8d1dda544fe862b9efb0e5345eeab5aab
Signed-off-by: Marc Jones <marc.jones@se-eng.com>
Reviewed-on: http://review.coreboot.org/804
Tested-by: build bot (Jenkins)
Reviewed-by: Peter Stuge <peter@stuge.se>