I'd rather avoid the duplication, this is discussed in #707.

Add missing fexecve() - execute program specified via file descriptor.

Reference: http://pubs.opengroup.org/onlinepubs/9699919799/functions/fexecve.html

Also move them to the common Linux module, as they are not platform-specific.
See include/uapi/linux/falloc.h.

There were previous definitions of SYS_gettid and SYS_perf_event_open

These are all that remain for nix to compile on s390x

It needs to be skipped during testing because there's no way to include
a combination of headers that will expose it without giving a "type has
already been defined" error.

We can't test s390x because qemu segfaults but we can at least verify that it
compiles.

Closes #650