platform: Fix compile error caused by standard includes

Avoid using standard includes namely stdint.h, string.h, stdlib.h, etc.

All standard include except stddef.h give compilation error due
to differences in RISC-V toolchain configuration. Typically, the
compilation error is related to "gnu-stubs-lp64.h".

This patch fixes compile error caused by standard includes by
providing substitutes of definetions provided by standard includes
wherever required.

Signed-off-by: Anup Patel <anup.patel@wdc.com>