sys: util: cast input of KB() to size_t for large number

Numbers in C macros are treated as signed integer. This causes
issues when using macro KB(x). If the result is larger than
(2^31 - 1), it is treated as a negative number downstream.
For example, the RAM size of up_squared is 2GB. The result of
KB(CONFIG_SRAM_SIZE) should be 2147483648, but due to being
casted as signed, the result is instead -2147483648. Any
calculations using this would be incorrectly. Fix it by
casting x in KB(x) to size_t first.

Fixes #27164

Signed-off-by: Daniel Leung <daniel.leung@intel.com>