add SS_AUTODISARM sigaltstack ss_flags from linux v4.7 to signal.h
add SS_AUTODISARM sigaltstack ss_flags from linux v4.7 to signal.h

only matters if swapcontext is used in a signal handler running on an
altstack, new in linux commit 2a74213838104a41588d86fd5e8d344972891ace

chardi/musl,chardi/musl,chardi/musl