gccgo: fix runtime compilation on NetBSD
si_code in siginfo_t is a macro on NetBSD, not a member of the
struct itself, so add a C trampoline for receiving its value.
Also replace references to mos.waitsemacount with the replacement and
add some helpers from os_netbsd.go in the GC repository.
Reviewed-by: Ian Lance Taylor <firstname.lastname@example.org>
4 files changed