ARCMWDT: Fix issues with posix types

Fix posix pid_t type redifiniton (zephyr vs arcmwdtlib).
Add omitted in arcmwdtlib type sigevent.

This Fixes https://github.com/zephyrproject-rtos/zephyr/issues/75105



Signed-off-by: Nikolay Agishev <agishev@synopsys.com>