glibc 2.12 support

[Florian Weimer]

It seems that building against glibc 2.12 is still supported.  Is this
something that is still needed?

I'm mostly concerned with this fallback code on x86-64:

// Unfortunately we have to bring all these macros here from vsyscall.h
// to be able to compile on old linuxes.
  #define __NR_vgetcpu 2
  #define VSYSCALL_START (-10UL << 20)
  #define VSYSCALL_SIZE 1024
  #define VSYSCALL_ADDR(vsyscall_nr) (VSYSCALL_START+VSYSCALL_SIZE*(vsyscall_nr))
  typedef long (*vgetcpu_t)(unsigned int *cpu, unsigned int *node, unsigned long *tcache);
  vgetcpu_t vgetcpu = (vgetcpu_t)VSYSCALL_ADDR(__NR_vgetcpu);
  retval = vgetcpu(&cpu, NULL, NULL);

There is no way to check that the kernel actually supports vsyscall, and
on some kernels, this will crash because they have disabled vsyscall.

I would like to remove this or switch over to the system call (as
already used on i386).  This is fallback code only, so performance does
not really matter: on newer glibc (starting with 2.14), sched_getcpu
will be found, and it will use vDSO or rseq as appropriate.

Thanks,
Florian