Use glibc's memfd_create() when available
Use glibc's memfd_create() when available

Summary:
Originally memfd_create() did not exist in glibc, so code had to do its own
raw syscall to invoke it. It was added as a proper function in glibc 2.27,
so prefer it when available.

Reviewed By: mroch

Differential Revision: D21333706

fbshipit-source-id: 3f260e8db6f56a637a4db32b7b7ddb0c38382721

facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow