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>

 reporter_name="Remy Bohmer"

 reporter_url="linux@bohmer.net"

 reporter_comment="Example for building a toolchain for m68000 bare metal targets"