Hi,

while doing a glibc trunk build with host i686-pc-linux-gnu and target
mips-linux-gnu, I ran into the following build failure:
...
configure:6203: checking for -z relro option
configure:6273: error: linker with -z relro support required
...

This error originates from sysdeps/alpha/preconfigure, that contains some
settings that aren't conditioned on the machine being alpha*.

Fixed by:
- moving the setting of libc_cv_gcc_unwind_find_fde to
  sysdeps/unix/sysv/linux/alpha/configure.in.
- making the setting of libc_commonpagesize and
  libc_relro_required conditional on the machine being alpha*.

Tested by finishing the MIPS build.

Thanks,
- Tom