guix: compile glibc without -werror
guix: compile glibc without -werror

Compiling glibc 2.24 and 2.27 with the new GCC 10 results in a number of new warnings,
i.e:
```bash
libc-tls.c: In function ‘__libc_setup_tls’:
libc-tls.c:208:30: error: array subscript 1 is outside the bounds of an interior zero-length array ‘struct dtv_slotinfo[0]’ [-Werror=zero-length-bounds]
  208 |   static_slotinfo.si.slotinfo[1].map = main_map;
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from ../sysdeps/x86_64/ldsodefs.h:54,
                 from ../sysdeps/gnu/ldsodefs.h:46,
                 from ../sysdeps/unix/sysv/linux/ldsodefs.h:25,
                 from libc-tls.c:20:
../sysdeps/generic/ldsodefs.h:398:7: note: while referencing ‘slotinfo’
  398 |     } slotinfo[0];
      |       ^~~~~~~~
```

While we could try and backport all the patches required to fix these up, it would
currently seem easier to disable -Werror, which Guix uses by default when building
glibc.

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