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>

# ARM specific configuration file

config ARCH_arm

    select ARCH_SUPPORTS_32

    select ARCH_DEFAULT_32

    select ARCH_SUPPORTS_BOTH_MMU

    select ARCH_DEFAULT_HAS_MMU

    select ARCH_SUPPORTS_BOTH_ENDIAN

    select ARCH_DEFAULT_LE

    select ARCH_SUPPORT_ARCH

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    select ARCH_SUPPORT_FPU

    help

      The ARM architecture, as defined by:

        http://www.arm.com/