libc: hide no-thread for those C libraries that do not support it

Building non-threaded glibc has been unsupported for a long time, now:
http://sourceware.org/ml/libc-alpha/2005-08/msg00091.html

As eglibc is a spin-off of glibc: ditto.

So do not offer that possibility in the menuconfig.
Thanks to Thomas Petazzoni for spotting, and helping to solve, the issue!

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# x86 specific options

config ARCH_x86

    select ARCH_SUPPORTS_32

    select ARCH_SUPPORTS_64

    select ARCH_DEFAULT_32

    select ARCH_USE_MMU

    select ARCH_SUPPORT_ARCH

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    help

      The x86 architecture, as defined by:

        32-bit (ia32) : http://www.intel.com/

        64-bit (amd64): http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_875_7044,00.html