Use SIGRTMAX - 2 instead of __SIGRTMIN + 2 on non-bionic linux

Only bionic reserves __SIGRTMIN + 2 for use by libcore, on other
linux platforms it must use something between SIGRTMIN and SIGRTMAX.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native -k
Change-Id: I19bb066e94dab102a7e3ada5ce2a7fcd994f1101
3 files changed