Fix vdso system call fallback failures.

When a vdso call falls back to making a regular system call, the inline
code for the system call doesn't know about errno and just leaves the
usual kernel result to be translated. Add the missing translation.

Also fix the defaults for non-vdso systems so we actually take the
fallback path (and so avoid unintentionally doing the errno translation
twice in those cases).

Bug: http://b/69626243
Test: ran new tests from http://b/63737556
Change-Id: If379632ea2e059e3d3bc3ff41bf3608dc05fb0a3