[PATCH] Linux: Add memfd_create system call wrapper

[Joseph Myers]

This breaks the testsuite build for hppa, which does not use mman-linux.h 
(i.e. the new declarations also need to be added to the hppa bits/mman.h).

../sysdeps/unix/sysv/linux/tst-memfd_create.c: In function 'do_test':
../sysdeps/unix/sysv/linux/tst-memfd_create.c:70:20: error: 'MFD_CLOEXEC' undeclared (first use in this function); did you mean 'FD_CLOEXEC'?
           flags |= MFD_CLOEXEC;
                    ^~~~~~~~~~~
                    FD_CLOEXEC
../sysdeps/unix/sysv/linux/tst-memfd_create.c:70:20: note: each undeclared identifier is reported only once for each function it appears in
../sysdeps/unix/sysv/linux/tst-memfd_create.c:72:20: error: 'MFD_ALLOW_SEALING' undeclared (first use in this function); did you mean '_PC_ALLOC_SIZE_MIN'?
           flags |= MFD_ALLOW_SEALING;
                    ^~~~~~~~~~~~~~~~~
                    _PC_ALLOC_SIZE_MIN
../sysdeps/unix/sysv/linux/tst-memfd_create.c:75:18: error: implicit declaration of function 'memfd_create' [-Werror=implicit-function-declaration]
         int fd = memfd_create ("tst-memfd_create", flags);
                  ^~~~~~~~~~~~
cc1: all warnings being treated as errors

https://sourceware.org/ml/libc-testresults/2017-q4/msg00352.html

(With non-mainline GCC the build failure still appears, it's just not so 
visible as a regression because previously the testsuite build would 
complete but with test failures.)

-- 
Joseph S. Myers
joseph@codesourcery.com