syscall: support sparc64 (and maybe other big-endian systems)
Thanks to the GCC compile farm project, we can wire up syscalls
for sparc64 and set system-specific SFD_* constants properly.
I've FINALLY figured out how to use POSIX::SigSet to generate
a usable buffer for the syscall perlfunc. This is required
for endian-neutral behavior and relevant to sparc64, at least.
There's no need for signalfd-related stuff to be constants,
either. signalfd initialization is never a hot path and a stub
subroutine for constants uses several KB of memory in the
interpreter.
We'll drop the needless SEEK_CUR import while we're importing
O_NONBLOCK, too.