Unlike libc syscall stubs, ld.so syscall stubs are expected to return -errno,
rather than -1, upon failure.

Even though nothing in ld.so currently needs to explicitely check for specific
error values, consistency doesn't hurt.
Add more sugar to generate syscall stubs; and make sure errors are reported
with a negative value (I'm looking at you, _dl_mmap);
Add more sugar to build syscall stubs, and turn them from inlines to real
entry points, now that we no longer need _dl_mprotect to be inline; also,
make sure syscall failures return -errno rather than -1.
Add more sugar to build syscall stubs, and turn them from inlines to real
entry points; also, make sure syscall failures return -errno rather than -1.