syscalls: Define the ST_* constants and add 'statfs-flags->mount-flags'.
* guix/build/syscalls.scm (linux?): New variable.
(define-statfs-flags): New macro.
(ST_RDONLY, ST_NOSUID, ST_NODEV, ST_NOEXEC, ST_SYNCHRONOUS)
(ST_MANDLOCK, ST_WRITE, ST_APPEND, ST_IMMUTABLE, ST_NOATIME)
(ST_NODIRATIME, ST_RELATIME): New variables.
(statfs-flags->mount-flags): New procedure.