libc/glibc: set the endian option according to extra CFLAGS

When building a multilib, some extra CFLAGS can override the
default config option. This is the case for the endianness
selection.

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

# Compute Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

}