os.rs: fix wrong syscall number for PowerPC
os.rs: fix wrong syscall number for PowerPC

__NR_getrandom is 359 on PowerPC, added from torvalds/linux@7d59deb50aa v3.17-rc5.
The bug was introduced from 05f23d275 3 years ago, affecting from 0.1.1 to before 0.5.0-pre.0.
It has already been fixed on the 0.5 branch at 00713a61c so this is a backport.
rust-lang/rand