x64: Make DUMMY_HANDLER less dumb

It's nice to at least see the RIP where the invalid opcode exception
happened at...

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
x64: 'syscall' instruction support

Enable "fast system calls" via the 'syscall' instruction on OSv. The
instruction is used by Go programs on Linux/x86-64 for system calls.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
linux: Make syscall() return ENOSYS if syscall not supported

Make the syscall() function return ENOSYS if a syscall number is not
supported. Keep a debug printout, though, as our system call coverage is
very low.

This is needed for statically linked programs to limp along even if a
syscall isn't supported.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>