gpt: The leXX_to_int() calls replaced with ones defined at <compiler.h>

Custom definitions of le_XX_to_int functions have been replaced with
standard ones, defined at <compiler.h>

Replacement of several GPT related structures members with ones
indicating its endianness and proper size.

Signed-off-by: Chang Hyun Park <heartinpiece@outlook.com>
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>