libc/glibc: LinuxThreads are no longer supported in latest versions

In fact, it is only supported in a few legacy versions.

Keep LT available for all eglibc versions, although it might need
a similar safeguard...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 0c0734c5d62ca4e09f5d54be4fe7f5c97bfc7539)

# Blackfin specific configuration file

config ARCH_blackfin

    select ARCH_SUPPORTS_32

    select ARCH_DEFAULT_32

    select ARCH_DEFAULT_LE

    select ARCH_SUPPORT_ARCH

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    select ARCH_SUPPORT_FPU

     help

      The Blackfin architecture