Define SYS_(pread|pwrite) as __NR_\1 on glibc systems unless defined by glibc

Define SYS_(pread|pwrite) as __NR_\1 on glibc systems unless defined by glibc
itself.

This solution is _very_ugly_, but it's the only way I know which doesn't force
people to rebuild the glibc every few kernel releases. Grrr. I don't understand
why glibc uses its own include for syscall numbers and doesn't just include
<asm/unistd.h> instead.