This commit changes the way the list of SYS_* system call macros is
created on Linux. glibc now contains a list of all known system
calls, and the generated <bits/syscall.h> file defines the SYS_ macro
only if the correspnding __NR_ macro is defined by the kernel headers.
As a result, glibc does not have to be rebuilt to pick up system calls
if the glibc sources already know about them. This means that glibc
can be built with older kernel headers, and if the installed kernel
headers are upgraded afterwards, additional SYS_ macros become
available as long as glibc has a record for those system calls.
