UAPI: Fix the guards on various asm/unistd.h files
asm-generic/unistd.h and a number of asm/unistd.h files have been given
reinclusion guards that allow the guard to be overridden if __SYSCALL is
defined. Unfortunately, these files define __SYSCALL and don't undefine it
when they've finished with it, thus rendering the guard ineffective.
The reason for this override is to allow the file to be #included multiple
times with different settings on __SYSCALL for purposes like generating syscall
tables.