syscall: get rid of custom syscall implementation
    
    Maintaining the custom system call code for each architecture
    is a burden.  I'm not convinced that we have clobber registers
    correct, and at best, we are just duplicating work that has
    already been done by the glibc maintainers.
    
    I asked Ben what the reasoning was behind not just using syscall.
    This was his answer:
    
     The main issue was that glibc's pthreads implementation really
     sucked back during initial development and there was a use-case
     for having the io_XXX functions usable directly from clone()ed
     threads that didn't have all the glibc pthread state setup for
     per-cpu areas to handle per-thread errno.  That made sense back
     then, but is rather silly today.
    
    This patch gets rid of the architecture specific system call
    wrappers in favor of using syscall().  We leave the per-arch
    header files with the syscall numbers because we'll be adding
    a new system call soon, and this will allow us to add the
    new number without requiring newer kernels on build servers.
    
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>