Fix -Wconversion warnings in os code, primarily linux

Fix Wconversion warnings coming from these files:
37 /src/hotspot/os/linux/os_linux.cpp
6 /src/hotspot/os/posix/os_posix.cpp
5 /src/hotspot/os/linux/attachListener_linux.cpp
4 /src/hotspot/os/linux/os_perf_linux.cpp
2 /src/hotspot/os/posix/signals_posix.cpp
1 /src/hotspot/os/linux/waitBarrier_linux.cpp
1 /src/hotspot/os/linux/systemMemoryBarrier_linux.cpp
1 /src/hotspot/os/linux/cgroupSubsystem_linux.cpp