Fix build break when SYS_memfd_create doesn't exist

In memfd_create() if SYS_memfd_create is not defined then uname and
flag are unused and gcc complains.
Fix powerpc build on Ubuntu

In commit df1508cd4151 "move rtas syscall definition to ppc specific
directory" the rtas syscall got moved to syscalls/ppc, and we added a
rule in the Makefile to pick that up based on the output of
gcc -dumpmachine.

That rule only looks for "ppc*" though, on Ubuntu gcc emits
"powerpc-linux-gnu", or "powerpc64le-linux-gnu".