improvements to memfd_create

* move test from std/io/test.zig to std/os/test.zig
 * do glibc version check, and make direct system call if
   glibc is too old
 * disable test when not linking libc, to avoid not working
   with outdated qemu version on the CI server. see #4019
8186211404Andrew Kelley committed on 12/31/2019, 9:34:14 PMparenta153a97
3 files changedLine totals unavailable